BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF Hettrg M. Sage 1S91 (^JmuSiL 3777 Cornell University Library QE 262.P12R35 1910 The geology of the country a'o""jJ Padsto 3 1924 004 553 149 Cornell University Library The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924004553149 MEIOIES OF THE GEOLOaiCAL STJEVEY ENGLAND AND WALES. EXPLANATION OF SHEETS 335 & 336. THE GEOLOGY OF THE COUNTRY AROUND PADSTOW AND CAMELFOED CLEMENT REID, F.E.S., G. BARROW, F.G.S., AND HENRY DEWEY, F.G.S. WITH CONTRIBUTIONS BT ^ J.S. FLETT, M.A., D.Sc., and D. A. MacALISTER, A.E.S.M. i PUBLISHED BT OBDEE OF THE LORD OOHMISSIONEBS OF HIS MAJBSTI'S lEEASUBT. LONDON : PRINTED FOR HIS MAJESTY'S STATIONERY OFFICE^ By DAEHNG & SON, Ltd., 34-40, BACON STREET, E. And to be pmchased from E. STANFORD, 12, 13, and 14, Long Acre, London ; W & A. K. JOHNSTON, Ltd., 2, St. Andrew Squabe, Edinbubgh ; HODGES, FIGGIS, & Co., Gbafton Stbbet, Dublin ; From any Agent for the sale of Ordnance Survey Maps ; or through any Booker Irom T. FISHER UNWIN, 1, Adelphi Teeeaob, London, W.O., -who is the sole Wholesale Agent to the Trade outside the County of London. 1910. Price Two ShilUnga and Threepence, GEOLOGICAL SURVEY AND MUSEUM. (Office : 28, Jekmyn Steeet, liONDON, S.W.) PUBLICATIONS RELATING TO THE SURVEY OP CORNWALL AND DEVONSHIRE. MAPS. GENERAL MAP ON THE SCALE OP i INCH=1 MILE (1 to 253440). Sheet 18 (with part of 17), which includes North Devonshire and large parts of Somersetshire, Monmouthshire and Glamorganshire, Sheet 21 (with Sheet 25), which covers nearly all Cornwall, with the exception of a small part in the east of the County, and Sheet 22, which takes in part of Bast Cornwall, Central and South Devonshire and part of South West Dorsetshire, are pub- lished, price 2s. 6d. each. ONE-INCH MAPS, NEW SERIES, ON THE SCALE OP 1 INCH=1 MILE (1 to 63360). T"or prices of these maps, see next page of cover. Sheet 311 ("Wellington and Chard) 1906 ; Explanatory Memoir, price Is. 3d. (1906). Sheet 325 (Exeter) 1899 ; Explanatory Memoir, price 2s. (1902). Sheet 326 with 340 (Sidmonth and Lyme Regis). 1906; Explanatory Memoir, price Is. (1906). Sheet 335 (Trevose Head) 1910 ; > _ , ^ „ ■ ,1n1n^ Sheet:336 (Camelford) 1910 ; } ^-Pl-^t-y Memoir (1910). Sheet 339 (Newton Abbot) 1899 ; Explanatory Memoir in preparation. Sheet 346 (Newquay) 1906 ; Explanatory Memoir, price 3s. (1906). Sheet 347 (Bodmm and St. Austell) 1909 ; Explanatory Memoir, price 4s. (1909). Sheet 348 (Plymouth and Liskeard) 1907 ; Explanatory Memoir, price Bs. (1907). Sheet 349 (Plymouth, and Ivybridge) 1899 ; Explanatory Memoir in preparation. Sheet 350 (Torquay) 1898) ; Explanatory Memoir, price 2s. (1903). Sheet 351 with 358 (Land's End District) 1907 ; Explanatory Memoir, price 3s. 6d. (1907). Sheet 352 (Fahnouth, Truro and Camborne) 1906; Explanatory Memoir, price 7s. 6d. (1906). Sheet 353 with 354 (Mevagissey) 1907 ; Explanatory Memoir, price 2s. (1907). Sheet 355 (Kingsbridge) 1908 ■> „ , ^ „ „,^ Sheet 356 (Start Point) 1898 j^^planatory Memoir, pnoe Is. 6d. (1904). Sheet 357 with 360 (Isles of Scilly) 1906 ; Explanatory Memoir, price Is. (1906). Sheets 337, 338, 359, with Explanatory Memoirs, are in preparation. MEIOIES OF THE GEOLOGICAL STJEVEI ENGLAND AND WALES. EXPLANATION OF SHEETS 335 & 336. THE GEOLOGY OF THE COUNTRY AROUND PADSTOW AND CAMELFORD BY CLEMENT REID, F.R.S., G. BAEROW, F.G.S., AND HENRY DEWEY, F.G.S. WITH CONTRIBUTIONS BY J. S. FLBTT, M.A., D.Sc, and D. A. MacALISTEB, A.R.8.M. PUBLISHED BY ORDER OF THE LORD OOMMISSIONEES OF HIS MAJESTY'S TREASURY. LONDON : PRINTED FOR HIS MAJESTY'S STATIONERY OFFICE, By darling & SOX, Ltd., 34-40, BACON STRERT, E. And to be purchased from ' B. STANFORD, 12, 13, and 14, Long Acbb, London ; W. & A. K. JOHNSTON, Ltd., 2, St. Andrew Square, EDiKBURrxH ; HODGES, FIGGIS, & Co., Grafton Street, Dublin ; From any Agent for the sale of Ordnance Survey Maps ; or through any Bookseller from T. FISHER UNWIN, 1, Adelphi Terrace, London, W.C, who is the sole Wholesale Agent to the Trade outside the County of London. 1910. Price Two Shillings and Threepence. ev.i Ill PREFACE. . Tliis Memoir is an explanation of Sheets 335 and 336 of the New Series Geological Map on a scale of one-inch to one-mile. The original survey of the district was made by Sir H. T. De la Beche and the results were published in 1839 on the Old Series One-inch Sheet 30. In the earlier issues of this Sheet the sedimentary rocks were classed as Grauwacke, but in the later editions they were coloured as Devonian. The new maps are based on a re-survey on the six-inch scale by Messrs. Reid, Barrow and Dewey, under the general superin- tendence of Mr. Reid as District Geologist. Mr. Reid has sur- veyed the area south of the Camel, Mr. Barrow the granite of Bodmin Moor and most of its contact-aureole, while Mr. Dewey is responsible for the rest of the area. Mineral lodes have been laid down from observations in the field by the officers concerned, with some additions from old mine plans by Mr. MacAlister. In this Memoir the field-officers have described the areas for which they are severally responsible. The petrology has been contributed by Messrs. Barrow and Dewey with assistance from Dr. Flett. Mr. MacAlister has supplied some of the mining information, and Dr. Ivor Thomas has identified the fossils. On comparing the old and new maps several important differences will be observed. The Devonian rocks have been separated into Upper, Middle and Lower, the " greenstone " outcrops have been considerably modified, the contemporaneous volcanic rocks have been separated from the more massive green- stones, the elvans are no longer represented as apophyses of the granite and much information as to the nature and distribu- tion of the rocks of the contact-aureole has been added. During the progress of the survey of this region considerable additions were made to the collection of photographs illustrative of the geological features, rock-types and scenery. Of these a few have been reproduced in the Memoir; for the remainder, reference should be made to the Catalogue of Photographs (Mem. Geol. Stirvey) or to the special list on p. 111. J. J. H. TEALL, Director. Geological Survey Office, 28, Termvn Street, London, 12th September, 1910. (15999—17.) Wt. 3587H— 34. 000. 12/10. D & S. IV List of the Six-Inch Geological Maps included in the area. Of these MS. coloured copies are deposited for public reference in the Library of the Geological Survey and Museum of Practical Geology ; or they can be supplied at the cost of drawing and colouring. Cornwall. 10. NW., NB., SW., SE. by H. Dewey. 11. SW. by H. Dewey. 13. SB., by C. Reid. 14. NW., by H. Dewey ; 14. NB., by H. Dewey and G. Barrow ; 14. SW., by C. Keid and H. Dewey ; 14. SE., by G. Barrow and H. Dewey. 15. NW., by H. Dewey and G. Barrow ; 15. NE., by H. Dewey ; 15. SW., by G. Barrow ; 15. SB., by G. Barrow and H. Dewey. 18. NW. and NB., by C. Reid and H. Dewey ; 18. SW., by C. Reid ; 18. SE. by C. Reid and H. Dewey. 19. NW., by C. Reid and H. Dewey ; 19. NE., by C. Reid ; 19. SW., by H. Dewey ; 19. SE., by H. Dewey. 20. NW., by C. Reid and H. Dewey ; 20. NE., by G. Barrow ; 20. SW., by H. Dewey ; 20. SE., by G. Barrow. 21. NW., NB., SW., SB., by G. Barrow. 24. NW., by C. Reid ; 24. NE., by C. Reid and H. Dewey ; 24. SW., SE., by C. Reid. 25. NW., by C. Reid and H. Dewey ; 25. NE., by H. Dewey ; 25. SW., by C. Reid ; 25. SE., by 0. Reid and H. Dewey. 26. NW., by G. Barrow and H. Dewey ; 26. NE., by G. Barrow and H. Dewey ; 26. SW., by G. Barrow and H. Dewey ; 26. SE., by G. Barrow. 27. NW., NE., SW., SE., by G. Barrow. 32. NW., NE. (parts of), by C. Reid. 33. NW., NB. (parts of), by G. Barrow. 34. NW. (parts of), by G. Barrow ; NE. (parts of), by W. A. B. Ussher 35. NW., NE. (parts of), by W. A. E. Ussher. CONTENTS. Table of Strata, Literature Preface by the Director Chapter I. — iNTKODncxiON. Chapter II. — Devonian — Lower Devonian ... ... ... Middle Devonian Chapter III.— Devonian— co";.— Upper Devonian Altered Rocks of the Ntirthern Area ... ... Chapter IV.— Devonian Volcanic Rocks and Greenstones Volcanic Rocks Petrography , Grreenstones of the Southern Area Epidiorites Sheared Granitic Rock Alteration caused by Greenstone Chapter V. — Granite and later Intrusive Rocks — Granite Finer Granite Granite Veins Modifications of the Granite Elvans Minettes Chapter VI. — Aureole op Thermometamorphism surrounding Granite — The altered Killas Calc-flinta Flinty biotite hornfels, or Leptynolite Sheared Adinoles Pneumatolitic Changes in the Killas Altered Greenstone Chapter VII. — Tertiary and Drift — Tertiary Raised Beach River Terraces Stream Tin and Wolfram Deposits Peat Sand-banks and Dunes Coast Scenery Chapter VIII.— Economics. Building Stone Slates and Flags ... Road Metal China-clay Mining Output of Tin and Copper Output of Minerals other than Tin and Copper Index of Mines Stream-tin and Wolf ram Water Supply Appendix. — Bibliography Index Page. iii 1 17 28 36 40 42 47 47 48 51 55 56 56 58 61 63 69 72 73 74 75 78 80 83 85 88 88 89 90 91 93 93 99 104 105 105 107 108 109 113 fLLUSTRATIONS. Fig. 1. — Mother Ivey's Bay and Merope Rocks Fig. 2.— Marble Cliffs, Porthmissen Fig. 3. — Map showing the distribution of the Altered Rocks near Gamelford Fig. 4. — Section showing the relation of the Stream-tin deposits to the Ancient Valley Fig. 5. — Diagram to illustrate Mode of Working a Clay-pit Fig. 6.— Pawton Mine Fig. 7.— Treburgett Mine and Alluvial Page. 19 21 29 86 98 101 102 Plate I. — Banded Slate of Trebetheric Point. Plate II. — Greenstone Dyke and Altered Slate at Stepper Point. Plate III.— Granite. Plate IV. — Photomicrographs. . § THE GEOLOGY OF THE COUNTRY AROUND PAD8T0W AND CAMELFORD. CHAPTER 1. lA'TRODUCTIUN. The following pages describe the geology of that part of Corn- wall which stretches from Bodmin Moor westward to the sea, and includes the towns of Padstow, Wadebridge, and Camelford. More exactly it may be considered as taking in two-thirds of Bodmin Moor, most of the basin of the Camel, and the coast from Mawgan Porth to Boscastle. Most of this area is included in Sheets 335 and 336 of the Geological Survey Map ; but the annexa- tion of a small area around Tintagel (in Sheet 322) enables us to complete the description of the Upper Devonian rocks which run in and out across the boundary. The total area is about 200 square miles. The surface contours of this district are decidedly complex and difficult to describe in a few words. On the east we find the high granitic Bodmin Moors, which rise to 1,375 feet at Brown Willy and exceed 1,000 feet at many points. But these high moors do not form the main watershed, which is found further north, off the granite and on slate, at a height of about 900 feet. This elevated slate ridge stretches westward to the coast at Tintagel, and then turns southward, at a somewhat lower elevation, run- ning near and parallel to the coast. Thus most of the tributaries of the Camel flow southward, but the main river is suddenly deflected by the high ridge of St. Breock Downs (700 feet) and turns north-westward, to reach the sea at Padstow. Though the basin of the Camel occupies most of the area, and is mainly included within the area, much of Bodmin Moor drains southward towards the Fowey ; a few small streams flc w eastward to join the Tamar, and others, mainly south-west of Padstow, reach the sea by a direct course over a comparatively low west- ward-sloping plateau. Mining, or rather streaming, was at one time a great industry on Bodmin Moor, much tin occurring in the alluvial deposits, though lodes are small and scattered. The streaming ceased for a series of years, but lately there has been a revival, especially in those alhivial deposits wliich happen to contain wolfram, formerly of no value. A few tin and copper mines are still being worked. Over the slate and lava areas antimony has been mined at different times ; but the lodes are small and impersistent. Iron has also been mined. 2 GEOLOGY or PADSTOW. Tlie most important mineral industries of the district are now the workings for granite, slate, and china-clay. In the mam it is an agricultural district, with rough pasture on the granite, where many ponies are bred, itnd also on the high slate and grit areas. The soil over the slates is often very thin, but it is better and deeper where volcanic rocks occur. The strong winds, combined with the south-westerly slope of most of the area, make it more suitable for dairy farming than for tillage. Woodlands are confined to the sheltered valleys some miles from the coast. The geological formations represented in Sheets 335 and 336 are as follows : — Sedimentary. f Blown sand. Recent. •{ Peat. 1^ Alluvium. _ / Vallev G-ravel and Head. Pleistocene. ^^ ^^.J^ ^^^^^ f Upper Devonian slate. I Do. limestone. Devonian. Greenstone (intrusive). Devonian. \proterobase J Upper / Pillow-lava (spilite). Devonian. \ Tuff and Schalstein. In addition to these we find various types of metamorphic rock in the neighbourhood of the igneous masses. No strata of earlier date than Lower Devonian have yet been reached in the district. The ores found are those of tin, tungsten, copper, silver, lead, antimony, iron, manganese, and zinc. The geological structure of this region is somewhat complicated. The dominant strike of the Devonian rocks is the usual east-and- west strike of Cornwall and Devon ; but this is modified by various synclines and anticlines, the axes of which have been tilted by the subsequent intrusion of the large granite mass of Bodmin Moor. In addition to this tilt, we have evidence that a .prolonga- tion of the granite mass, or another buried dome stretching north- westward, has so raised the strata locally as to give a north- westerly strike along our northern border. Notwithstanding these complications, the east-and-west strike is the dominant feature. The cleavage in the slates is usually well marked and tends to coincide with the bedding ; but over extensive areas which happen to be free from hard igneous rocks the cleavage-planes are nearly horizontal, or merely undulate slightly. Where, however, massive gieenstone is approached the cleavage usually tilts up at high angles. In one part, near the great dyke of Stepper Point, which has an exceptional strike, the Devonian rocks seem INTEODUCTION. 3 to liave been protected from cleavage by this massive vertical rib, and the bedding planes in tlie killas almt against the dyke without showing any perceptible cleavage. The other features of the district will be more conveniently dealt with when we have described the geology. Literature. Though there are some excellent published descriptions of parts of our district, the geological literature is not extensive, and con- sists largely of references to mines and minerals.^ Perhaps the first observation on .the geology is that recorded in 1818 by Sir Humphrey Davy, who noted the occurrence of mandelstein or wacke near Port Isaac. After this, except a reference to the use of shell-sand, nothing appeared until 1832, when Dr. H. S. Boase accurately described the lithological character of the rocks of the w^hole area, but did not refer to the geological structure. In 1839 De la Beche's geological map. Sheet 30, and also his ' Eeport on the Geology of Cornwall, Devon, and West Somerset,' were published. These show the first attempt to work out the geological structure of this part of Cornwall. The older strata were divided into C'arbonaceous Deposits and Grau- wacke Group, the latter showing further belts of contemporaneous igneous rock. and bands of limestone. The main belts of green- stone were sketched out. The granite boundary was mapped in detail, and this rock was shown as sending intrusive tongues or elvan dykes into the slaty rock, or killas as it is locally termed. This was a great advance on anything that had been done before ; but owing to the confusion in those days between true dip and mere dip of the limb of a fold, the succession in the grauwacke was not understood. Later editions of the map introduced the term Devonian, showed the aureole of metamorphism caused by the granite, and added more mining information. Daniel Sharpe in 1847 referred especially to the slaty cleavage in dealing with the northern part of the district and noted the difference between the dip of the cleavage and of the bedding in the slate. Pillowy forms in the lavas of Pentire were described in 1848 by N. Whitley as being like " bales of cloth." He also figured the pillows. Sedgw-ick in 1852 called attention to the way the sediments wrapped round the north end of the protruding boss of granite. S. R. Pattison in 1865 noted the succession of rocks in the area north of Bodiuin Moor, and suggested an underground exteii- sion of a spur from the granite mass, giving rise to the dome structure. J. Arthur Phillips added considerably to our knowledge of the petrology and gave analyses and figures of many of the rocks in two papeiK published in 1875 and 1878. This petrological work A\a& continued by Dr. J. .1. H. Teall in 1888 in his ' British Petrography,' in which are descjibed several igneous rocks from the Padstow district. ' Titles of the more important books and papers relating to the district will be found in the Appendix (p. 100). 4: GEOLOGY OF PADSTOW. Tlie altered rocks of Tintagel were investigated in 1889 by- Mr. W. M. Hutcliings, and by Mr. Parkinson in 1903, the latter separating them into different belts or stratigraphical horizons according to their mineralogical character. Probably the greatest advance since De la Beche's day in our knowledge of the geology of this region we owe to the steady work of Mr. Howard Fox, who year after year has examined different parts of the coast, and has obtained the assistance of the best palaeontologists and petrologists in the determination of his specimens. The long list of his papers will be found in the Appendix to this Memoir ; his work must be referred to again and again, and it is therefore unnecessary here more fully to refer to it. Of those who have worked with him we may especially mention Dr. Henry Woodward, Dr. G. J. Hinde, Dr. F. A. Bather, Mr. Crick, and Mr. Walter Barratt. The other publications of recent date relating to the district are mainly by officers of the Survey, and give accounts of some of the discoveries made in the progress of the work. Their titles, and the titles of various other papers not mentioned in the above paragraphs, will be found in the Appendix. CHAPTER II. DEVONIAN. A general northward dip brings on jiewer deposits in tliat direction, until near Boscastle the Upper Devonian rocks dis- appear beneath Culm Measures. The lowest beds are found on the coast at Trenance, near our southern border; they are the Lower Devonian slates known as»the Meadfoot Beds. These are followed northward by more silty and sandy strata (Staddon Grit) in which occur thin bands of cherty sandstone or quartzite. These sandy strata form the uplands of St. Breock Downs, and as the sandstone, though only occurring in thin beds, is almost indestructible, the whole of this area is dotted over with standing stones, stone circles, and scattered blocks. The occurrence of these conspicuous blocks tends to give the impression that these downs are essentially formed of hard sandstone ; but this is not the case, for wherever sections are visible we find that hard sand- stones are rare and thin, the thickest being only four feet, though occasionally bands are found close together. On the north side of the Staddon Grit ridge the country falls rapidly to a plain sloping towards the river Camel. This much- dissected plain is formed by the grey Middle Devonian slates, in which the only variations met with consist of a few thin seams of limestone, some silty bands, and a few intrusive sills. The transition from Middle to Upper Devonian is almost imperceptible; but the incoming of the newer strata was marked by volcanic outbursts, culminating in the thick mass of pillow- lava which forms the headland at Pentire. The Upper Devonian rocks, unlike the Middle Devonian, are very variable. They are grey in the lower part ; but grey slates soon give place to purple and green slates, and these, again, to green silty slates with thin seams of grit. In the lower part of the series there is also a definite calcareous horizon, with thin limestones so numerous as to form a conspicuous feature in Marble Cliff (Fig. 2). This limestone series is diiiicult to trace inland, tor thin limestones tend to disappear at the oiitcrop through solution by rain-water: but where this belt enters the aureole of metamorphism around the granite it becomes even more conspicuous than on the coast, where the calcareous rocks have been converted into insioluble lime-silicates, they form striking masses of calc-flinta. LowEH Devonian. Lower Devonian rocks occupy a few square miles only of the area under consideration, and they include only the upper parts of the division. On the coast near Trenance the top beds of the Meadfoot Series are seen on the shores and in the cliffs of Mawgan Porth. They consist of dark-grey non-calcareous shale, weathering green near the joints, and containing casts of crinoid ossicles and of a spiral gasteropod, probably Pleurotomaria . Closer to the Staddon Grit these pass into hard, platy, dark-grey 6 GEOLOGY or PADSTOW. slate with thin, pale-green seams, the latter probably repre- senting fine volcanic dust. These slates either alternate with oi are contorted into the base of the Staddon Grit, but the junction is not easy to interpret, and is accessible only at extreme low- water. The Staddon Grit in the clifE north of Mawgan Forth lies m flattened S-shaped folds of alternating coarse grit and gritty slate; but the bulk of the material consists of gritty slate. Towards its upper limit this division seems to pass ini- perceptibly into sandy thin-bedded silt; but the whole clifi is so contorted, and is so difficult of access, that it is impossible to say anything definite as to the relations with the beds above and below. The only fossil yet discovered in any of these sandy deposits, or in the slates associated with them, is a tail of a trilobite (Homalonotus) discovered by Mr. Walter Barratt as these pages were passing through the press. The inland exposures of Staddon Grit call for little remark. If the Little Petherick stream is followed upwards towards Little Pennatillies it will be found to _ pass from grey slate to hard grit, the first of the transverse ribs of grit having been used as the foundation of the dam for the mill-pond. Old mine-shafts close to the mill show, however, nothing but dark-grey slate. Thus we seem to have a fairly definite northern boundary for the grit series at this point. If we follow the crest of the ridge from the coast inland, we find that the smooth surface rises gently as far as Trevilledor. Then we meet with a comparatively steep bluff running north and south, at right angles to the strike of the underlying con- torted rocks. This bluff has nothing to do with the underground structure, it is the old Pliocene cliff-line, which has been traced here and there almost from end to end of Cornwall at the same level ; here, as usual, its foot is about 430 feet above the sea; it will be again referred to. Above this bluff the contours become bolder and the land is rougher and more strewn with large blocks. Just below the bluff two quarries will be found, in strata inaccessible on the coast. One of the quarries lies close to the tumulus, shown on the map south-west of Trevilledor, the other touches the 400-foot contour just ovitside the map. Both quarries are in flaggy grit and grit-shale. The open heaths, over which the Newquay road passes, look almost like granite moors, they are so covered with large scat- tered blocks. Unfortunately, these blocks are rapidly being broken up for road-making, and the peculiar charm of this wild heath is disappearing. Even the standing stones, cromlechs, and stone circles tend to suffer unless carefully protected. These erections, it may be remarked, are always formed of the local rock, probably not moved many yards except by natural agencies. On the higher part of these downs there are no quarries, but one will be found at Haycrock, where the micaceous flaggy sand- stone seems to belong nearly to the top of the Staddon Grit. In this neighbourhood it is, however, very difficult to draw a satis- factory boundary between Lower and Middle Devonian, for large blocks of grit have travelled long distances down the gently- sloping plateau which extends to the Camel. This travelling LOWER DEVONIAN. 7 of tlie blocks tends greatly to mask the junction ; it is due to former arctic conditions and will again be referred to iu Chapter VII. A mile east of Haycrock, at the point where the high road enters the open heath, a quarry in pink and green silty slate has been con- sidered to belong to the lower part of the Middle Devonian ; but it may equally well be part of the Staddon Grit without hard bands. East of this point the grit bands tend to die out and silty strata become more prominent. In the area between Grogley Downs and Bodmin fine sections are exposed in the transvCTse valleys and railway cuttings, but the upper part only of the Staddon Grit group is met with, and the beds possess the same characters as in the area to the south and south-east, described in the Bodmin and St. Austell Memoir. The group consists of a number of thin bands of grit or grey- wacke, usually hard and compact if fresh, dispersed through a much thicker mass of killas originally of finer, though variable, texture. There are three well-marked types of killas, dis- tinguished by the clearness of their original bedding or banding. In the first the alternations of fine, pale, sandy material and grey mud are easily visible to the unaided eye, and in these a minute puckering is always clear. In the second, a hand lens is often required to make out the bedding, the rock being essentially a very fine silt. Though splitting more easily along parallel planes, the rock does not possess a good cleavage, and can be dressed by an axe across these planes. It is, apparently, f el- spathic, soft, and very friable. The third type is not visibly banded ; it approaches more to a slate, but it does not cleave into thin plates; like the second type, it can be cut or hacked across the splitting-planes when freshly quarried. When built into walls it sets and hardens and forms a fairly durable building stone, many of the houses at the west end of Bodmin being built of this material. It is more siliceous than a typical Middle Devonian slate (described later), and probably some of the silica is present in a soluble form, which would account for its becoming markedly harder when dry. All three of these types, when built into walls, or exposed in a position where they can dry, assume a decidedly pink tinge. This distinguishes them from the killas well up in the Middle Devonian, which does not assume this colour on drying. Numerous sections of the rocks above described occur near the railway between Bodmin and Grogley Halt (half-way to Wadebridge). To the east of the first halt at Dunmere the out- crop of the highest grit band is exposed for some yards; the rock is a pale greenish greywacke, and the sand-grains can be seen by the unaided eye; the pinkish weathering associated with it can also be examined here. Many exposures are seen between this point and the junction of the two railways at Bos- came. Here a high rock-face shows a number of the little grey- wacke bands, which vary considerably in tint, owing to the increase or decrease of the amount of chlorite in them. The peculiar lenticular or ' thrust-plane ' structure, developed in such rocks by powerful regional movements, is present in prac- tically oil the bands, and good specimens showing it can be obtained here. For some distance the exposures are poor and 8 GEOLOGY OF PADSTOW. few grit bands are seen; but at Grog-ley Halt, wbere the two valleys unite, there is a specially good illustration of the violent contortion and fracturing to which the grits have been sub- jected. The fractures are filled with vein quartz, which permeates a considerable portion of the rocks here exposed, and has invaded the grit bands, partially replacing their original constituents. The section is useful as showing the origin of the great quantity of vein quartz lying loose about the fields. This is specially abundant in areas formed of the Staddon Grits; their more siliceous nature clearly assisted in ike formation of a greater amount of this material. As regards the age of the strata just described, we can say little, for the few fossils found in the Meadfoot Beds of Mawgan Forth are so compressed, sheared, and altered as no longer to be specifically determinable. The beds, however, are continuous with slates undoubtedly of Lower Devonian age. The grits are even more uncertain, for in this district they have yielded only one fossil. In Devonshire, the Staddon Grits, apparently on the same horizon, yield a small Lower Devonian fauna. Middle Devonian. iliddle Devonian slates occupy a belt of ground between Park Head and St. Neot, apparently expanding northward to St. Breward. But though occupying so large an area the rocks are singularly uniform in character, consisting of grey slate and occasional thin seams of limestone. The sections, except in the cliffs, are of little interest, and even on the coast it is not easy to find characteristic fossils, though the strata are usually full of badly-preserved impressions. ISTo sign of contemporaneous volcanic activity has yet been observed in these slates, though the junction with the Upper Devonian has been taken to coin- cide with a well-marked band of volcanic ash. Thus far it has not been found possible to subdivide the Middle Devonian slates of Cornwall into palteontological zones, for though charaeteristio Middle Devonian species have been found, these fossils, except the anomalous Pteroconiis m.irus Hinde, do not seem to be con- fined to particular horizons. As the coast sections are good and clear, we will describe them first, afterwards indicating the probable inland equivalents of each division. The Middle Devonian of the coast has been examined and described by Mr. Howard Fox,' to whom and to Mr. Walter Barratt we are indebted for the discovery of most of the fossils. In the following notes we have made free use of this work, as we cannot add greatly to the published descriptions. The junction of the Lower and Middle Devonian is best seen from the beach below, which can only be approached by- descending Bedruthan Steps and then turning southward along the shore. Pendarves Island can be rounded at low-water; but at other states of the tide the southern beach can only be reached by climbing the narrow neck which connects the island with ' ' Geological Notes,' Trans. R. Geol. Soc. Cornwall, vol. xii (1899), pp. 347_ 354 ; ' On the Distribution of Fossils on the North Coast of Cornwall south of the Camel.' Ibid. (1901), pp. 535-544, MIDDLE DEVONIAN. -determined by Dr. G. J. Hinde. the land. This is a difficult scramble, and at any time may become quite impracticable; great caution is therefore needed, as there is no other way of escape from the bay beyond, and at half-tide escape is entirely cut off. On reaching the bay and looking up at the vertical cliffs, which rise for about 300 feet, it is still impossible to make out the nature of the junction ; for the enormous flat-lying folds are so broken by faults and thrusts that we can only say that on the south are seen grit beds, while on the north these give place to hard ferruginous shale with crinoids. On repassing Pendarves Island we find immediately dark slate, full of Pteroeonus mirus Hinde, forming sloping ledges on the foreshore. This fossil seems to have so limited a range in the lower part of the Middle Devonian that it ought to be useful for zonal purposes ; but its zoological position is still in dispute ; it is not associated with any other characteristic forms, and it is unknown elsewhere. The species recorded by Mr. Howard Fox are as follows : — Petraia sp. ■ Zaphrentis calceoloides ? Champ. Zaphrentis sp Campophyllum ? Lopnophyllum ? Pleurodiotyum sp. Michelinia sp Favosites sp Alveolites sp Pachypora reticulata (Blainv. Aulopora sp .. SphaerocrinuB sp. Bhodocrinus or Melocrinus Scaphiociinns ? .•> Acanthocriaus sp. Palasteriscus devonicus Stiir Serpulites? Phacops sp Polypora ripisteria {Goldf.) Fistulipora sp. Monticuliporoid Retepora sp Chonetes sp Orthis arcuata Pliill. ... Spirifera sp. Murchisonia sp. Loxonema sp Pteroeonus mirus Hinde Orthoceras af. cochleiferum G. & P. S'andb. Pteraspis sp Scaphaspis sp Climatius? G-anoid Fucoids ? • J ) ••1. determined by Rev. G. F. Whidborne. determined by Dr. G. J. Hinde. ^determined by Dr. F. A. Bather. fej ■> determined by Dr. G. J. Hinde. determined by Rev. G. F. Whidborne. .. !- determined by Dr. G. J. Hinde. ..J ., determined by R. B. Newton. -) ■■ > determined by Rev. G. F. Whidborne. .. determined by R. B. Newton. .. determined by Rev. G. F. Whidborne. .. determined by Dr. G. J. Hinde. determined by G. C Crick. " [-Determined by Dr. A. Smith Woodward. .. Determined by Dr. G. J. Hinde. To this list we can only add Alveolites verviicvlaris, recorded by McCoy, an indeterminable species of Ncdicopsis and a doubtful Mimoeeras; it seems, therefore, that notwithstanding the large number of genera represented, it is quite impossible at present to indicate the exact horizon or zone to which the fossils belong. 10 GEOLOGY OF PADRTOW. Most of the calcareous fossils in this part of the Middle Devonian were destroyed before the slate was cleaved, so that nothing- was left but an impression more or less filled with pyrites. Subsequent cleavage and shearing has so damaged these impressions as to render the determination of the species most difficult. To the north of Bedruthan Steps Pteroconus is found in a quarry at the edge of the cliff on the north side of Red Cliff Castle, and in the isolated stack below, known as Samaritan Island. This part of the shore provides good walking at low tide, past Queen Bess rock as far as Diggory's Island, where Pleurodictyum occurs. These cliffs show grey or greenish-grey contorted slates, un- dulating, and also much disturbed after the cleavage was developed. The next bay can be reached by Pentire Steps, at its northern horn. The rocks are grey slates with pyritized fossils. At the foot of the steps, on the foreshore, will be found a small mass of greenstone, which appears to be a continuation of the larger outcrop in Park Head. It is not visible, however, in the cliff near Pentire Steps, and must be either the nose of a folded sill, or is brought in by a west-north-west fault. The bay between Pentire Steps and Park Head is inaccessible from above, and very dangerous for a boat. Park Head itself is formed by a hard mass of greenstone (quartz-diabase) much mixed with slate, so that their mutual relations are not clear. The next cove on the north is in- accessible; but Lower Butter Cove, which is a quarter of a mile north of Park Head, shows a V-shaped mass of greenstpne in the roof of a cave. This cove is in fossiliferous grey slate, in which Mr. Howard Pox found: — Petraia sp. Pteroconus mirus Hinde Pleurodictyum sp. Oonularia sp. Crinoid ossicles we noticed in addition an Orthoceras and the tail of a trilobite. North of Park Head, as soon as we get beyond the influence of the massive greenstone, the flat-lying cleavage, already referred to, becomes very conspicuous, but the rocks exhibit no other change till Porth Mear is reached. Here the grey slates show thin seams of limestone and of hard grit, the fossils being crinoid stems, Pleurodictyum, and simple corals. At the head of this Porth the true bedding is vertical, with a west-south- west strike. It is seldom possible to walk from Porth Mear to Porthcothaa along the shore, and Trescore Islands are best approached from the north, at low-tide, from Porthcothan. The cliffs of Porth- cothan exhibit excellent sections, which are easy of access. The grey slates show numerous impressions of fossils, amongst which trilobites are fairly plentiful. In the cliff oppos'ite Trescore Island Mr. Walter Barratt discovered Phacops lati- frons'? (Bronn).' From Porthcothan Beach M'e can record ' The species described by Dr. Henry Woodward as Homalonntun harratti came from the Upper Devonian of Mother Ivey's Bay, not from Porthcothan as recorded. MIDDLE DEVONIAN. 11 tfle following, those with an asterisk being added in the course of the Geological Survey and determined by Dr. Ivor Thomas : — Petraia oeltica Lonsd. Orthoceras sp. II sp. *ActinoceraB sp. Pleurodiotyum sp. Goniatites sp. Crinoid . * Asteropyge punctata ? {Salter) ^0 tenodonta ? [non Stein.] trastropod *cf. Phacops granulatus {Munst.) Conularia complanata Slater, var. * „ latifrons (Bronn) *Tentaculites sp. * „ sp. The small cove known as Boathouse Cove, on the south side of Porthcothan, yielded a good specimen of Orthoceras robertsi Whidb., one of the few characteristic Middle Devonian fossils found on this coast. Beyond Minnows Islands the character of the slate changes somewhat, and the rocks are shown on the map as Upper Devonian. The colour is still grey, but there is a tendency to conspicuous fine banding; many seams are also crowded with a small tentaculite, unfortunately never sufficiently well preserved for specific deter- mination. Thin interrupted bands of black fine-grained grit con- stantly occur, and also scattered nodules of black phosphate, the latter frequently containing Conularia. In other respects the fauna is poor and little varied; so that it is impossible to say definitely whether these striped slates should be. regarded as Middle Devonian, or whether they ought to be considered as the base of the Upper division. As a matter of convenience, and in default of direct evidence, the line has been drawn at the first appearance of contemporaneous volcanic activity, though unfor- tunately ihe conspicuous band of tuff, traced for several miles inland, is lost near St. Merryn Church. It seems to be shifted to the south by a north-north-west fault. It is very thin at Trevorgus and has not been found on the coast. Inland Exposures. Little need be said about the inland exposures of the Middle Devonian in this area, for though the slates are commonly f ossili- ferous, no characteristic fossils have yet been found. Road- cuttings show the slates everywhere in the region between the coast and Wadebridge ; but they call for no comment beyond the statement that the slates are always grey and show well the striking flatness of the cleavage-planes ._ We will briefly describe the best inland sections, commencing with the oldest strata, which lie close to the margin of the Staddon Grit. A mile inland from Park Head, at Treburrick, large flagstones of poor quality have been quarried, the cleavage lying flat and here coinciding with the bedding. Other similar quarries will bo found north of Penrose and near Trevebban, and there is a large and deep quarry north-east of Pawton in hard flaggy slate. This belt of flagstones is perhaps equivalent to the slate which appears in the inaccessible cliffs close to Park Head. No fossils were observed on this horizon, and the absence of fossils and of calcareous matter makes the slate split more evenly and stand the weather better than most parts of the Middle Devonian. 15999 B 12 GEOLOGY OF PADSTOW. The Wadebfidge and Newquay road climbs the downs by a small ridge and watershed, which seems connected, with the curious fold shown on the map. The presence of this fold makes it difficult to say Tiow near to the Staddon Grit any horizon may be; but it is noticeable that we find within a mile of the grit several sections that remind us of the highly fossiliferous strata near Bedruthan Steps. The first of the sections will be found close to No Man's Land, in an old quarry just above the stream. It shows grey gritty slate, weathering pale green-grey, full of fossils, mainly crinoid remains and simple corals. The only ones that Dr. Ivor Thomas could determine . were Combophyllum?, Favositesf, and Goniophora? Half a mile higher up this stream another quarry will be found in this green-grey fossiliferous sandy slate, and still further, half a mile south of No Man's Land, grey thick-bedded fossiliferous slate is worked. The tip from the shaft of Pawton Mine, half a mile east-north-east of the same place, shows hard grey slate with crinoid ossicles and cleavage crossing the bedding planes. These sections were carefully searched for Pteroconus, but without success. It should be borne in mind, however, that in the case of pyritised fossils the sea clears away the matrix in a manner never seen in inland sections. Outside the pyrites there is usually a hard silicified crust which is removed by the drifting sand, but when the slates are split one usually sees only this crust or a broken piece of pyrites. Near the surface also the pyrites usually decomposes, and then, even if the mould is not destroyed, one only finds a nodule, hollow inside, but with the hollow so filled with limonite or ochre as to be scarcely recognisable. These quarries, however, are worth further search, and being cFose to the main coach-road are easy of access. The quarry at the point where the high-road enters the open heath, a mile east of Haycrock, has already been referred to. It shows pink and green silty slate, which might belong either to the Staddon Grit or to the base of the Middle Devonian. It yields no fossils. East of Wadebridge the Middle Devonian outcrop narrows and then expands greatly ; so that while the junction with the Staddon Grit continues to follow the usual east and west strike, the northern boundary swings round to the north, till it abuts against the granite near St. Breward. This area is much disturbed and the slate is probably folded and faulted in such a way as to make the correlation of sub-divisions almost hopeless. In addition to this, the metamorphism caused by the granite is sufficient to mask for at least a mile from the granite margin the slight lithological distinctions which characterise the different belts of unaltered sediment. Petrological zones are developed, depending to a great extent on proximity to the granite, but almost equally on differences of chemical com- position so slight as to be almost unnoticeable in the unaltered rock. To add to these difficulties, there is a decided increase in the amount of lime towards Plymouth, and this is not noticed in the ordinary decalcified inland sections, though" it becomes con- spicuous inthe metamorphic aureole, where the lime is locked up in tjie form of insoluble silicates. MIDDLE DEVONIAN. 13 In addition to the exposures already described there is a possible inlier of Middle Devonian south-east of Padstow, between Dennis and Cant. It is difficult to understand by what combination of anticline and fault Middle Devonian can be brought up in this position, and as the fossils are not decisive, it has been thought best for the present to include these slates in the Upper Devonian. This doubtful area is described on pp. 22, 24, 35, and 39. In the area south-east of Wadebridge, immediately above the Staddon Grit, a series of hard silty grey-green beds, weathering rusty brown is highly fossiliferous. These rocks, probably the same as those seen at No Man's Land, are typically exposed at Trerice, south of the farm buildings, and beds with similar crinoids and corals are quarried in Hustyn Woods. Banded grey slates succeed to the north and are exposed in road-cuttings near Burlorne Eglos and in Bishop's Wood. Near Polbrook Bridge they are thrown into violent contortions. Eastward of this point they are replaced by pink and grey-green silty and sandy sedi- ments, which appear in the fields in large flaggy and cuboidal pieces. This type occupies all the area between Bozion and the river Camel, north of Dunmere, but it does not suffer the change of strike described above as affecting the Middle Devonian rocks. These silty rocks bear a strong resemblance to the top part of the Lower Devonian ; if they belong to this division their position must be due to an anticlinal fold pitching to the west, or else to a north-north-west fault following this long straight reach of the Camel valley. Good sections of the rocks of this division occur between Grogley Halt and Polbrook. West of the Halt are several large quarries, the first of which is in the hard grey fairly homogeneous killas. Though the material splits readily along one plane, it does not possess a good cleavage, the splitting being uneven. It differs markedly from the good slates higher in the series, in that it contains more finely divided quartz and the proportion of white micaceous material to chlorite is smaller. It probably contains a fair amount of soluble silica, for it becomes much harder when thoroughly dried. The second quarry is in the banded silt, consisting of alternations of pale siliceous and grey aluminous material ; the bands are usually of equal thickness, roughly about one-eighth of an inch. This banding serves to bring out the persistent minute puckering characteristic of these beds, the puckered banding having an un- usually high dip to the north, while the puckers are traversed by a strain-cleavage having a low dip in the same direction. In many cases the rocks have been affected by a movement parallel to the axis of the puckering, thus showing the rodding structure often met with in crystalline schists. This peculiar structure causes the material to weather into small splinters, much longer than broad, which when seen lying loose in the fields have the aspect of broken and decayed sticks. About two hundred yards north of the quarry just referred to, numerous loose fragments of greenish-grey killas are to be seen on the east side of the railway. These contain abundant crinoid ossicles, some of which are very large and are elliptical in form. 15999 B 2 14 GEOLOGY OF PADSTOW. Further on a little band of siliceous material occurs, passing rapidly on one side into a specially liard grey killas, wliich in turn passes into grey killas of the same nature as that seen in the first quarry. From this point to the cottage of Polbrook there are alternations of finely-banded and homogeneous grey hard killas ; but if they be compared with those already described, there is a distinctly greyer tint in the specimens as a whole, and the siliceous element is slowly giving way to the more aluminous. This is specially well shown in some banded puckered rock close to the cottage ; in place of the bands of different material being of equal thickness, the sand occurs as mere films and the films are at unequal distances apart. The recognisable siliceous material steadily dies away, till at three hundred yards north of the cottage we enter on the great homogeneous mass of grey slate, charac- teristic of the Middle Devonian. At this point, however, we have not yet reached the fine silky highly fissile slates, which are higher in the sequence. Microscopic sections of rocks of this group confirm what is seen in the field ; even in the finest-grained of these passage-rocks little clastic flakes of white mica are rarely absent. Furthermore, in the lower, harder, less cleavable parts of the main mass of the Middle Devonian slate minute flakes of clastic white mica are usually present, but they are very rare in and iisually absent from the clean-cleaving slates. They were probably very small originally and for the most part have been completely digested. A specially interesting exposure of the puckered banded material occurs in a quarry in Dunmere Wood, some five hundred yards north of the bridge over the Camel. Here also the banding has a high dip to the north and the puckers are traversed by a strain- slip cleavage which has a low dip in the same direction. Many fossils occur, though in the solid rock their edges only are faintly visible; but in the waste material on the alluvial ground outside the wood there is much decomposed rock from which a consider- able number of fossils were obtained. These, however, proved to be so distorted as to be indeterminable. Further north, both on the west side of the Camel and at times iu its bed, good sections are exposed, especially about the mill- dam. Here are seen a few thin siliceous green bands, not coarse enough to be claimed as grits. Similar fine-grained thin siliceous bands are common on the coast about the middle of the Middle Devonian. Thin patches of calcareous killas also occur, and the gradual darkening of all types of beds as we ascend in the series is clearly shown. The more aluminous and main portion of the Middle Devonian can be well studied in many continuous sections about the mineral railway, along the bottom of the Camel valley, to Wenford- bridge. At the foot of Clerken water faint traces of puckering and banding are still visible to the unaided eye, but further north the rocks become more homogeneous and of a slaty aspect. For some distance the grey killas is too siliceous and hard to possess a good clean cleavage, but beyond Pencarrow and Colquite Woods the rocks steadily become softer and more of a true slate. Near Hellandbridge several large quarries have been opened in these slates to supply building material. MIDDLE DEVONIAN. 15 A wide area to the soutli-east of St. Mabyn, between the village and the river Camel, is occupied by killas varying from pale-grey to glossy-black in colour, and as the granite is approached the effect of contact metamorphism is pronounced. The slates take on a satiny sheen, tend to lose their cleavage, and often approach mica schists in appearance; but no definite contact minerals have arisen in this outer zone of metamorphism. Well-cleaved grey slates are quarried at Spittal and near Foley's Bridge, but do not call for special description. In the section of the line between the Blisland valley and Wenfordbridge the slates cleave readily into thin flakes, have a silky aspect and very dark cross-fracture. The preservation of their dark colour is due to their occurrence at the bottom of a valley which has recently been deepened, owing to the Pliocene uplift. _ On the high ground above they are often much paler. This original dark colour is equally well retained in the deep valleys to the south of Cardinham. There a number of exposures of the Middle Devonian slates occur, and a change may be noted, as we ascend in the series, from the hard, grey, slaty rock to the fine fissile slate with very dark cross fracture. In the lower part of the valleys below Warleggon and St. Neot this latter dark slate with perfect cleavage alone occurs, the lower beds having passed out of the area here described, owing to the south- easterly strike of the rocks. Quarries in this part of the Devonian Rocks are numerous; most occur on the edges of the steep valleys, where erosion goes on fast enough to remove the decomposing material to a con- siderable extent, so that here the grey colour is largely main- tained. The rock is extracted for building; only in the valley below St. Neot has it been quarried for slate. Fossils are extremely scarce. Scattered crinoid ossicles are not uncommon in the lower part, but even these are rare in the upper. One fossiliferous horizon has been met with to the west of the Racecourse Downs, in a quarry on the east side of Holton Farm. In the south-eastern corner of the quarry a curious red- stained band may be traced for some yards in the face of the rock. A considerable number of fossils occurred in this, but, as usual, crinoid ossicles were the most abundant, and after this, specimens of more or less distorted corals. Mr. E. T. Newton recognised Petraia, Fenestella, and possibly Cyathocrinus. Directly in the line of strike of the beds in this quarry, a small band of impure limestone was found in the stream bed in Helland Wood ; it seems to be composed entirely of crinoid ossicles, and no other fossils could be found. The bed was exposed by an exceptionally heavy flood, and it is doubtful if it could be easily found again. Similar impure limestones have already been mentioned as occurring on the coast north of Park Head; but they da not appear in the lower part of the Middle Devonian. About a mile to the south-east of Holton, on the north edge of Ludcott Wood, is a singular outcrop of the banded killas, consisting of thin alternations of pale and dark grey material. The dip here can be seen to be high in a northerly direction, but on the main limb of the fold, little minor ' buckles,' about an 16 GEOLOGY OF PAD STOW. inch and a Jialf across, have been developed. The cut specimen shows perfectly the unequal yielding of the two materials; the softer dark mud being heaped up and the band thickened in the noses of the folds. A strain-slip cleavage crosses every alternate nose, i.e., the small concave folds facing in one direction are intersected, but not the convex ones facing the same way. Its occurrence in such a position is extraordinary, as it exactly resembles the passage beds seen far to the south; it is quite possible that it may be a portion of those beds brought up on the crest of a dome. 17 CHAPTER in. DEYONIAN— conimMed. TJpPEE Devonian. The Upper Devonian rocks are much more variable than the Middle Devonian, and in this district they also contain more fossils in a determinable state. We are able, therefore, to make some advance in their division into distinct palseontological zones, following on the pioneer work of Mr. Howard Fox and of Mr. Ussher, who has written so much on the Devonian strata of adjoining regions. One striking characteristic of these Upper Devonian rocks is the appearance of various belts of contemporaneous lava and volcanic tuff, which help greatly when we attempt to trace any particular bed across this much-disturbed country. In addition to this, different belts of slate vary considerably in their colour, texture, and lithological character. It should be easy under such conditions to map out the country into different zones, were it not that the disturbance is very great, and we have as yet no undisturbed type-section which can be, used as a standard for comparison. The order of succession made out in the following- pages must therefore be treated as to some extent tentative. An additional complication is caused by an area of meta- morphism, which, unfortunately, stretches right across the strike, from Bodmin Moor to the sea at Tintagel, thus cutting off all connection with the f ossilif erous strata of South Petherwin . We think that the Clymenia-heds of Petherwin- are newer than any Devonian rocks seen in the area now under consideration ; but even this is not certain. The most convenient way to deal with these rocks will be to describe first the coast section, beginning where we left off at the top of the Middle Devonian. We have already alluded to the band of volcanic tuff which has been taken provisionally as the junction between Middle and Upper Devonian. This tuff, which can be followed for about 12 miles, forms a conspicuous conical mound a quarter of a mile south of St. Merryn church; but we were unable to trace it more than a few yards further west. At the point where it is lost it seems to be cut by a north-north-west fault, forming a continuation of the Treveglos lead-lode. Half a mile further south, near Higher Trevorgus, it reappears on the west side of this fault ; but is very thin, and is only traceable for a quarter of a mile. It has not been found on the coast. The lowest Upper Devonian strata seen on the coast consist of finely-bandedj grey, silky slates, with seams crowded with a small tentaculite. Thin bands of fine-grained black grit are also noticeable, and scattered phosphatic nodules containing Contilaria are common ; there are few other fossils except occa- sional crinoid ossicles and simple corals, though two or three genera of indeterminable gastropods ^\■ere noticed. North of 4M GEOLOGY OF PADSTOW. Treyarnon Point these strata become very calcareous, and con- tain thin beds of limestone; but the conspicuous striping, the thin seams of black grit, and the phosphatic nodules continue to be the characteristic features, even in Booby's Bay, where undoubted Upper Devonian fossils are first met with. The slates of Booby's Bay deserve special attention. At the north angle of the bay, at the point where the sand-dunes end, there is much calcite in the slates. A quarter of a mile to the north-west, just south of a small spring which flows over the edge of the clifi, one band is full of a slender crinoid, some individuals having heads attached. We were unable to obtain a good specimen, for, unfortunately, the best seen was cut through from end to end by a thin vein of quartz. Immediately beyond this spring Mr. Howard Fox discovered a band full of small pyritous fossils, amongst which the characteristic Upper Devonian Buchiola retro striata (v. Buch) is abundant. _ As he had cleared away nearly all the visible fossils in this small exposure before the Survey arrived, his list will be given : — -determined by Dr. G. J. Hinde. Zaphrentis calceoloides ? Champ. „ sp Pachypora sp Criaoidal fragments . . , Phacops sp Braohiopods, indeterminate ... Buchiola retrostriata («. Buch) Tentaculites sp. Conularia complanata /SM«'', ««'•• ••■ determined by Miss Slater. Orthoceras aff. ooohleiferum G. & F. Sandb. Orthoceras sp }. determined by Mr. G. C. Crick. Bactntes sp '' Goniatites (Anarcestes) noeggerathi (v. Buch) North-west of this spring grey-striped slate full of Styliola and crinoids is seen, close to a conspicuous band of septarian nodules. Nodules of this sort have not been observed anywhere below this horizon; but somewhat similar masses have been found in the Treworder cutting of the Wadebridge and Launces- ton railway, and these may quite well be on the same horizon.^ After passing this septarian band in Booby's Bay, we come to a deep gully, which renders the foreshore impassable; but as far as can be seen from above, the cliff is still in striped grey slate as far as Round Hole. Dinas Head is so swept by wind and wave as to be nearly bare of soil. It shows several thin bands of limestone, which are probably equivalent to those seen in Marble Cliff, at Porthmissen. The limestones of Dinas Head contain some black chert with radiolaria, and at one place there are many silicified masses of PhilUpsastraea henriahi? (Lonadale),riiin.g\eA with Pachypora and a bryozoon. These striped highly calcareous rocks of Dinas Head ' H. Fox, Trans. R. Geol. Soc. Cornwall, 1895, p. 684. The Treworder nodules, however, show pitted surfaces and cone-in-cone structure, and under the microscope are seen to be built up of pleoohroio calcite (gee Bosenbusch ; Mik. Phys. Mineralien u. Gesteine, Bd. I, 2, p. 29). UPPEK DEVONIAN. 19 have been invaded by a massive greenstone sill or dyke, whicL. has altered them in a quite exceptional fashion and to a quite un- usual extent/ The dark-grey calcareous slates have been turned into a more or less laminated vrhite rock, with a conchoidal frac- ture and with a curious spheroidal structure. They contain much soda-felspar (albite), whilst the thin bands of massive limestone seem to be represented by bands of more coarsely spheroidal soda- rock without the lamination. The alteration extends for 50 or 100 feet from the greenstone, thus being much more extensive than the foot or two of adinole which usually accompanies the Cornish greenstones. A close examination in the field leaves no doubt that the whole of the soda-rock is an altered sediment, sharply defined from the intrusive igneous mass. The exposure is full of small faults and thrust planes, so that it is difficult to obtain exact measurements, or to trace any one thin band of lime- stone with absolute certainty from the unaltered to the altered state. The silicified corals would probably remain unaltered, but we have been unable to find this particular band in the adinole. Dinas Head is the best place in Cornwall for the study of the metamorphism caused by a basic sill. It should be compared with the area near Camelford, where calcareous rocks, probably of the same date as those of Dinas Head and Marble Cliff, have been invaded by a mass of granite, and have been turned into a rock of very different appearance — the so-called ' calc-flinta.'^ On crossing Trevose Head to Mother Ivey's Bay (Fig. 1) we meet with slates corresponding with those already described. Fig. 1. — Mother Ivey's Bay and Merope Bocks. I Howard Fox, ' On a Soda Felspar Bock at Dinas Head, North Coast of Cornwall,' Geol. Mag., 1895, pp. 13-20. » See below, pp. 69-72. 20 GEOLOGY OF PADSTOW. The north end of the bay shows hard dark-grey or striped slates, with a few seams of black grit, containing crinoids and Styliola. Mr. Fox records from this bay Centrpnella or Cryptonella, Hyolithes, Tentaoulites, Goniatites, and Mr. Barratt found Homalonotus harratti H. Woodw. The lime- stones do not reappear in these cliffs, but are again seen, associated with calcareous slate, on the east shore of Little Cove at the bottom of Mother Ivey's Bay. The outcrop of the limestone appears to have been shifted south by a fault; or it may be that the thin limestones of Little Cove and Cataclews Point are not so high in the series as those of Dinas Head ; they are so much disturbed by the intrusive proterobase as to render it difficult to make out their true position. This mass of proterobase, which yields the ' Cataclews-stone,' famed for its durability and use for fine carving, is very puzzling. It is intruded irregularly along the bedding planes, and at one point it looked as if it cut across the cleavage. It is cut off by a fault in Little Cove and cannot be found in Booby's Bay or at Porthmissen. The sill is much disturbed by shearing and thrusting. Harlyn Bay shows the same silty striped grey slate, with thin black seams and phosphatic nodules containing Conularia, that we have already described in Booby's Bay. Newtrain Bay is similar and is on the same strike. Trevone Bay, however, is further north and somewhat higher in the series, and here Mr. Howard Fox discovered a bed full of small cephalopods of the Biidesheim type. The rocks on the south side of Trevone Bay are calcareous grey slates with thin limestones, bedding and cleavage often coinciding. The fossiliferous ledges are those on the foreshore close to the stream, and the fossils are usually contained in small nodules or cakes of pyrites, exposed and cleaned by the scour of the sand. As Mr. Fox obtained far more species than we have been able to collect (it takes some years for fresh specimens to be exposed) we give his list^ : — Petraia celtica Orthoceras sp. „ sp. Bactrites biideBheimensis Amplezus sp. „ sp. Lophophyllum ? Tornoceras simplex Favosites sp. „ globosum Pachypora sp. „ sp. Crinoidal fragments Anarcestes cf latiseptatus Trimerooephalus laevis „ sp. Phacops sp. Mimoceras compressum Centronella or Cryptonella „ sp. Buohiola retrostriata Agoniatites transitorius ? Euomphalus sp. Ganoid Styliola sp. Fucoids ? Tentaoulites sp. This particular band of fossils has not yet been found elsewhere in Cornwall, and though it probably occurs at other points, it would be very difficult to find the small fossils, except where a considerable area of the bedding-planes is scoured by the sand ' ' On the Distribution of Fossils on the North Coast of Cornwall south of the Camel.' Tram. R. Geol. Soc. Cornwall, vol. xii (1901), pp. 535-544. UPPEB DEVONIAN. 21 between tide-marks. It probably occurs again in the vertical cliffs south of Cataclews Point and south of Dinas Head, but in neither case are there any accessible foreshore ledges, and at the top of the cliff pyritized fossils have generally disappeared. After passing the north horn of Trevone Bay, which is formed by a mass of proterobase cut off on the south by a fault, we cross a narrow belt of grey slate (quite inaccessible in the vertical cliff) and then reach the striking ' Marble Cliffs ' (Pig. 2) north-west of Porthmissen. Here alternating bands of limestone and slate occupy most of the vertical cliff, which is over 100 feet in height. On the north they are followed by striped grey slate, cleaved across the contortions, but there is no clear evidence that the unexpected thickness of the limestone is due to flat-lying folds.^ A little further north, near the south end of Guddrabridge Mine (the lead- lode marked on the map) there is seen a narrow belt of much Fig. 2.— Marble Cliffs, Porthmissen. decayed vesicular lava. This small outcrop is of great import- ance, for it is the first appearance of the pillow-lava which east- ward thickens into an enormous mass ; it also gives us a definite stratigraphical horizon from which to measure. A hundred yards north of this band of lava the character of the slate changes completely, grey slate giving place to alter- nating bands of purple and pale-green, but about the junction of the two colours there is seen some black slate, forming an in- accessible ledge on the shore. This may correspond with the black cephalopod-shale of Daymer Bay, which occurs in a similar position. ' Mr. Walter Barratt has given to the Survey Ecliinasterella sladeni Sturtz, found at Porthmissen and determined by Dr. Bather. 22 GEOLOGY OF PADSTOW. Near Butter Hole an enormous intrusive dyke of diabase has altered the slate for 80 feet on its south side and 50 feet on its north, but otherwise all the northern part of the Stepper peninsula consists of purple and pale-green slate. At Hawker's Cove dark slate is again seen, and a quarter of a mile further south dark-grey platy slate with small lumps of pyrites yielded an obscure fossil resembling a Buchiola. The purple slates continue in the cliff facing the Camel as far south as St. George's Well. Then we again meet with striped grey slate. St. Saviour's Point is formed by a continuation of the green- stone of Porthmissen; but it is noticeable that the limestone of Marble Cliff, traceable inland by the hollow formed where it has weathered away at the outcrop, gradually approaches this sill, and at Crugmeer the sill bends and seems to run across it. East of this point limestone cannot be traced north of the sill, but it reappears south of it on the shore immediately north of Pad- stow Harbour. As the Camel flows along the strike of the beds we have just described, and its estuary is bordered by low cliffs, it will be con- venient here to continue the description of these cliffs between Padstow and Wadebridge, returning by the north bank to the open sea. Or, if the harbour is crossed at Rock Ferry, the description of the sea-cliff will be found continued on p. 25. Half a mile south of Padstow two bands of tuff appear in the low cliff and two bands cap and encircle Dennis Hill. An out- lier of similar tuff caps Ball Hill. A mile and a half further east a thin vesicular lava makes its appearance in the southern cliff, thickens rapidly after crossing the river, where it is associated with tuff containing marine fossils, and within half a mile it is lost near Burniere. The tuff of Burniere is associated with undoubted Upper Devonian slates, but we have been unable to discover characteristic fossils associated with the tuffs of Cant Hill and Dennis. These latter may possibly be the basal tuff of the Upper Devonian, brought up by some faulting which we are unable at present to explain. Neither Mr. Tox's nor our fossils are conclusive either way, though they perhaps suggest that the grey slate below the tuff is the top of the Middle Devonian. The reefs on the shore outside Padstow Harbour yielded Conu- laria. Half a mile south of Padstow we found Petraia, Pleuro- dictyuTn problematicum (Goldf.), perhaps Actinopteria clathrata (Sandb.) and Conularia complanata Slater, var. Opposite Dennis House there is grey slate with black bands and phosphatic nodules ; this locality yielded Petraia, Pleurodictyum, a zaphrentid, crinoidal remains, Athyris?, Orthis?, a lamellibranch (perhaps Ctenodonta? antiqua), and an Orthoceras. The cliff on the east of Dennis Hill is in similar grey slate with black bands, yielding a number of fossils only with doubt generically determined. A few yards further south Orthoceras hercynicuTn? Kayser occurs. North of Tregonce the cliffs show grey platy slate with black bands. The fossils found north - f Ballhill were Petraia, crinoidal columnals, fAtrypa aspera (Schloth.), A. reticularis (Linn.), Chonetes hardrensis? (Phill.), Orthis?,ci. Orthotetes umhraculum (Schloth.), S^pirifer? These strata may represent the Middle TTPPEH DEVONIAN. 23 Devonian slates found near Minnows Island, but they are also like some in Booby's Bay. This question will again be referred to when we describe the north bank of the river, for there is no means of crossing in this neighbourhood and it will probably be most convenient to describe the sections in the order in which they are likely to be visited. The Camel Quarries, near Penquean, though now flooded, have been worked to a depth of 360 feet (300 feet below sea level). They show hard flaggy grey slate with a purplish tinge, but we could see no sign of limestone bands or of tuff in the waste. The slates were used for flagstones and for roofing slate of poor quality. East of Penquean the cliffs are very low, and the railway often masks them ; but as Styliola was noticed a quarter of a mile east of the road from Tregunna, we seem to be well up in the Upper Devonian; the rocks, however, are still grey slates, very like those just described as possibly Middle Devonian. Close to where Styliola was found we again meet with a thin band of lava, first seen in the railway-cutting and then at several points in the cliff. This lava is evidently the same band as that noticed near Padstow, and it is followed on the south in the same way by bands of limestone, formerly quarried in the little valley north of Trevanson. On the opposite side of the marsh, facing the old lime-kiln, will be found a small cliff of striped, shaly, and earthy limestone, full of fragmentary trilobites and Styliola; but we did not succeed in getting good fossils. From this point to Wadebridge the abandoned cliff is obscured, though here and there Styliola can be found. On crossing the river at Wadebridge and turning to the west, we again meet with the limestone in small exposures in the creek below Bodieve, and also in a decalcified form and as fossiliferous chert in the quarry in which greenstone is worked for road-metal. Then, on following the foreshore northward, we cross beauti- fully striped slate with Styliola, and come to an extremely curious section.' The volcanic rock there seen appears to be close to the limit of the large lava-flow, for though one band consists of highly vesicular lava, other parts are more like tuff, and con- tain occasional marine fossils. The deposit suggests the blowing to pieces of a submarine lava, rather than the fall of volcanic detritus ejected by a distant vent. This sheet thickens rapidly towards Burniere, where it disappears suddenly, cut off, apparently, by a fault. For 300 yards further north grey-banded slates continue.^ Then these give place to grey-green and purple beds for 200 yards along the shore. Low cliffs border the estuary of the Amble river on either side, those on the south consisting of grey slate; but the variegated series reappears on the northern shore near Trewornan House. Between the Amble river and Dinham Creek an alternation of variegated with banded grey slate occurs. In several localities these slates are fossiliferous, the chief forms being Orthoceras and Styliola. • Photo 455 in the Oeological Survey collection of English photographs, » Photos 451, 452, 454. 24 GEOLOGY OF PADSTOW. On either side of Treganna bridge numerous limestone bands occur in tke river. These are dark grey in colour and, apparently, unfossiliferous, although the grey slates which form the western side of the creek are full of Styliola. "West of Dinham Creek Mr. Howard Fox found in grey slate a perfect trilobite, identified by Dr. H. Woodward as Phacops granulatus (Miinst.). East of Cant Cove pale-grey slate, very soft and weathered, is crowded with fossils. It is associated with a narrow band of sheared tuff, and marks the incoming of the same slates of uncertain date that were found on the south bank of the river. This slate can be traced round Cant Hill to Gentle Jane. The following species have been recorded from it by Mr. Howard Fox^: — Petraia, 2 or more spp. Zapbrentis sp. Amplexus ? Cyathophyllum sp. Favosites sp. Alveolites sp. Pachypora sp. Pleurodictyum sp. Crinoid ossicles Phacops latifrons {Bronn) „ granulatus (^Miinst.) „ punctatus {Stein.) „ laevis ? {Miinst.) Montioulipora sp. Fenestella 2 spp. Polypora sp. Phyllopora sp. Orthotetes erenistria? (Phill.) ? Atrypa reticularis (Linn.) „ cf. desquamata /. de C. Sow. Spiriferq^. cultrijugatus C. F. Roem. „ of. specioBus {Sehloth.) Pentamerus brevirostris {Phill.) Rhynchonella laticosta Phill., var. ? Conularia subparallela Sandh. „ cf. deflexioosta Sandh. Orthoceras cf. hercynicum Kayier „ af. commutatum Oiebel Cophinooeras sp. Phragmoceras? Agoniatites sp. To this list we can add, from specimens collected in the course of the Survey : — Asteropyge punctata {Salter) [non Steiri.] Cbonetes hardrensis ? {Phill.) Orthotetes umbraculum {Schloth.) ? Atrypa aspera {Schloth.) Leptaena rbomboidalis {Wilck.), var. Cf. Nucula protei Miinst. Pteria sp. Actinopteria (? new species) CuouUaea unilateralis ? J. de G. Sow. Conularia complanata Slater, var. „ subtilis? Slater Poterioceras ? Between Gentle Jane and Porthilly Bay black-striped slate with nodules occurs, associated with grey and blue slate, varying from platy to papery. They contain numerous fossils, and in one place Pleurodictyum prohlematicuTn is very abundant. The nodules often contain Conularia. Near Porthilly Church the banded grey slates reappear, with Styliola, crinoids, etc., these beds containing cherty len tides and layers. Several bands of limestone, associated with a greenstone sill and resembling the masses already mentioned as occurring at Dinham Creek and Padstow, are seen on the north side of Port- hilly Bay. These limestone bands are of some extent, and can be traced for more than a niile inland, beneath the blown sand. Beds of dark grey slate interbedded with the limestone are crowded with small brachiopods, Styliola, and crinoids, and can be traced to the Crane Quay at Rock. • 'Further notes on the Devonian Rocks and Fossils in the Parish of St. Minver,' Trans, R. Geol. Soc, Cormoall, vol. xiii (1905), pp. 33-87. T7PPEH DEVONIAN. 25 . In order to explain the structure of the district it is now neces- sary to describe a small area a quarter of a mile from the coast, as the rocks there seen are hidden on the coast by sands. Several quiarries have been opened on the golf links and at Stoptide for hedging stone, and these supplied ashy slate and black tufi like the tuffs described as occurring near Burniere on the foreshore. There is a close resemblance between the rocks at the two locali- ties, and their relations to the surrounding strata. There is every reason to believe that they are the same beds repeated by folding. North of Rock quarry a wide stretch of sand dune is crossed by ridges of hard greenstone or porcellanised slate, but there is no cliff-section till we reach the small bay immediately north of Cassock Hill. Here banded grey slate, often nodular and con- cretionary, occupies the shore to a point near Bray Cottage, where purple and green slates, penetrated by an elvan dyke, are again seen. On the north side of Daymer Bay (the bay east of the Doom Bar) there is much of interest. The purple and green slates are highly f ossiliferous ; we noticed abundance of Entomis and Posi- donomya venusta, and collected exceptionally well-preserved Tentaculites teniiicinctus ? F. A. Roemer, as well as three trilo- bites, Phacops (Trimerocephalus) anophthalmus Freeh, P. lati- frons (Bronn), and a third species of the same genus. ^ Faulted against these purple and green slates, blue-black soft slate is seen ; it is rich in fossils, yielding Aulopora, Athyris ?, Chonetes hard- r.ensis (PhilL), Buchiola cf. semiimpressa Drevermann, Modiella pygrncea (Conr.), Orthoceras commutatuTn C. G. Giebel, Ortho- ceras sp., Tornoceras cf. auris (Quenst.). These fossils were determined by Dr. Ivor Thomas. Trebetherick Point is composed of beautifully banded purple, green and buff slates, and where the raised-beach platform is scoured free from drift these ribands of colour form conspicuous stripes along the shore (Plate 1). In Greenaway Cliff the cleavage- dip is variable and across the bedding; fossils are abundant. Continuous exposures of the variegated beds occur in the cliffs round Hayle or Polzeath Bay, and fossils are present in certain bands. On the south side of Pentireglaze Haven a strike-fault is well seen, but on either side of it are purple strata. At Pen- tire Haven, which is the small inlet running northward towards Pentire farm, striped nodular grey slate, with thin bands of limestone and shear-lenticles of vesicular lava, dips north at a high angle, and near this point the conglomerate of Gravel Caverns is encountered. The conglomerate consists of amygda- loidal lava, quartz and killas cemented with pyrites and contains pyritous casts of Buchiola retrostriata, etc. The grey- striped slates compose the cliff from this locality to the point where the pillow lavas appear. Thus the sequence already described as occurring in many localities now reappears in reversed order, showing that we have crossed a synclinal fold, • Similar fossiliferous sections will be found inland at Tredrizzick and Rooke Mill, 26 GEOLOGrY OP PADSTOW. within which the purple and green strata occupy the highest geological position. The pillow-lava, however, has thickened enormously on the north side of this syncline. Pentire Head must he more fully described in the chapter dealing with the volcanic rocks, so here we need only mention that the slate associated with the lava is striped grey, and contains Styliola and Tentacidites. A long stretch of coast showing this grey slate now succeeds, for the cliff runs almost parallel to the strike of the rocks, and minor folds bring in the same beds again and again. A series of small faults running in a north-north-westerly direction tends to shift the outcrops or break the continuity, so that the purple slate appears in the cliff of Downhedge Cove, east of Carnweather Point, and the lava outcrop at Trevan Head is shifted south- ward. It is only in a few cases, however, that we can see clearly what these faults do. Port Quin needs fuller description, for here we seem to meet with a small anticline which brings up beds slightly lower than any which we have been able to examine elsewhere on the coast east of Padstow Harbour. The entrance to Port Quin lies between crags of lava, on the south or lower side of which the slate is altered into a hard, white, splintery adinole. The first piece of this rock that was broken happened to contain a well- preserved trilobite, apparently an undescribed Proetus or Deche- nella, but, unfortunately, wanting the head. This discovery of a good fossil in a rock baked hard before the cleavage, and, therefore, not cleaved, made us search carefully for more; but no more could be found, except bad specimens of Siyliola. South of the lava and near the head of the harbour we meet with grey slate, somewhat lower in the succession. In this there are some half-tide ledges, mostly sparingly fossili- ferous, but on the east side, a few yards from the greenstone dyke, a band full of small pyritised fossils was discovered. The fossils of this band were collected and developed by Mrs. Reid, and Miss Grace Walker presented us with others. They proved to be of great interest, apparently belonging to a zone not before recognised in Britain, and yielding undescribed species. Dr. Ivor Thomas determines them as : — Phaoops (Trimerooephalua) pentops Orthoceras sp. /. Thomas Of. Tornoceras planilobum Allorisma concinna /. Thomas (G. & F. Sandb.) Buchiola retrostriata ? (v. Buch) Cf. Tornoceras globosum {MUnst.) Bellerophon (Pomatis ?) sp. Tornoceras cf. subundulatum Freeh Platyostoma sp. „ verneuili {Miinst.) Naticopsis spp. „ simplex ? («. Buch) It is noticeable that the new trilobite is a perfect roUed-up specimen, not as usual in the Devonian rocks a cast shell. The cephalopods are singularly small. The fauna suggests that of the ' Chiloceras-horizon,' first noticed near Brilon by Professor Kayser, who named the beds the ' Nehdener Schiefer." Several of the species found at Port Quin are new.^ ■ Zeittchr. d. d. geol. Oesellsch. Bd. xxv (1873) p. 602. » l70f Thomas, G^ol. Mag. 1909, pp. 97-102, UPPER DEVONIAN. 27 The black slates close to tke pillow-laya on this part of the coast ought to yield the Ort/iocera«-famia; of Daymer Bay; but there is great difficulty in finding half-tide ledges on this horizon and much of the cliff is quite inaccessible. Varley Head is composed of purple and green slate, let down by a north-west fault. On its east side the grey slate seems to pass underneath the purple in normal sequence. A continuation of this fault cuts across the various small headlands for a mile or two to the east, bringing in the purple slates again and again ; it is well seen on either side of Port Gaverne. We did not succeed in finding the cephalopod-bed of Port Quin either in Port Isaac or in Port Gaverne, and think that it probably lies below the sea-level. Half a mile east of Port Gaverne the black slates with Styliola give place to purple and green strata, the strike altering to east-south-east. If we still follow the coast, we now cross the strike of the strata, but find that the purple and green slates continue as far as the beginning of Bounds Cliff, sinking gradually under somewhat harder pale greenish-grey slate which alternates with thin bands of grit. We seem to have entered an area containing strata higher , in the series than any before mentioned. They are sparingly fossiliferous, though at the foot of Bounds Cliff obscure jirilobites, brachiopods, and crinoids were noticed. On either side of Dela- bole Point bands of salmon-colour appear in these pale slates. Between this and Crookmoyle Rock lenticles of lava make their appearance in the cliff and in an inland quarry on the road to Dinnabroad, another lenticle being found on the foreshore at Jacket's Point. The interpretation of the geological structure now becomes very difficult, and it is impossible to be certain whether, as we proceed northward, we are entering newer and newer strata, or whether we have crossed a syncline and are again meeting with the strata seen further south. In Trerubies Cove we find pale micaceous slate with bands of salmon-colour or brownish-red. Are these the purple and green slates again, so altered owing to their proximity to an area of metamorphism as to be recognised with difficulty ? Or are they newer than the pale slate with thin bands of grit just described? The fossils do not help, for they are not abundant, though one band at the south end of Tregardock Beach is full of a slender branching monticuliporid. Midway between the fossil-band just mentioned and the crag known as the Mountain the pale slate suddenly gives place to black slate in which is seen a thin sheet of lava. If this black slate, containing the band of lava, is the same as the black slate which lies below the purple and green near Padstow, it makes the general structure of the country fairly intelligible. We will, therefore, return to Padstow, pick up the bands of lava and lime- stone, and follow them inland along their strike. In this way only can we show the relation of the strata to the larger folds, and also to the uptilt caused by buried masses of granife"betweeii Bodmin Moor and the sea. It has already been shown that in the Padstow and Wadebridge area, beneath the purple and green slates, comes black slate iu 16999 e 2.8 GEOXQGY 01' PADSTQW. which is a sheet of contemporaneous pillow-lava and tufE. Some- what lower is found a belt of limestone. If we follow these two horizons inland, we find their outcrops somewhat displaced by faulting and thrusting; but they are clearly traceable to St. Mabyn, where the strike turns northward. The lava can then be followed round the St. Minver syncline to the sea, though the limestone is lost and probably does not come to the surface in the Endellion anticline. This Endellion anticline and a sub- sidiary one at Trequite bring up a belt of black slate and lava between two areas of purple slate. Following the northern out- crops of this lava, we find that the sheet is much broken, but its outcrop can be traced eastward to the railway, then swinging northward to St. Teath, where it becomes very thin. Parallel with "it we find within the aureole of metamorphism a mass of calcareous rock, stretching almost continuously from Michaelstow to Camelford. These two bands seem, therefore, to represent the two horizons which we have been following ; but the question arises : What happens to the lava north of St. Teath? The band becomes so thin there that we have not been able to trace it continuously; but it probably swings westward again, round another syncline, to reach the sea at Tregardock. There it passes out to sea, only to come in again at Trebarwith Strand. Prom that place it strikes northward to Tintagel and then turns suddenly eastward and southward, to re-enter our district at Tremail, in the north- east corner of the one-inch map {see fig. 3). If this correlation of the strata is correct, we ought again to find the OrthoceTas-hand. of Daymer Bay in the black slate of Tre- gardock Beach, and we might even find the cephalopod-bed of Port Quin, though it is doubtful whether any of the rocks on the coast at Tregardock are quite so low in the series. Altered Eocks of the Nokthern Area. We will now describe the area of altered rocks north of Lanteglos. The disposition of the beds will be seen by referring to the small- scale map given opposite. They occupy an area which is about 17 miles from east to west and 5 miles from north to south. Part only, about a third, of this area is included in Map 336, the remainder being in the sheet to the north (Boscastle, 322) and the one to the east (Tavistock, 337). The beds in descending order are as follows^ : — VI. Tredorn Phyllites. V. Trambley Cove Beds. IV. Volcanic Series. III. Barras Nose Beds. II. Woolgarden Phyllites. I. Delabole Slates. V ?Qnf !n^9 .^o«"^'*iy ^'■- Pa-^l^iason in the Quart. Journ. Oeol. Soc, yo , iK 1903, pp. 408^428, by Mr. Maynard Hutchings, Geol Mag. 1889, pp.53 265 280 ^ ''^^' ^""'■'' •^''"''"- ^^^- *"''•' ^°'- '^^-1909 29 s. 0? S •S C1 to 5» r=3 M 16999 C 2 30 GEOLOGY OF PADSTOW. I. Delabole Slates. — These are the well-cleaved, grey-green, and blue-grey slates which are quarried at the celebrated Delabole Quarries. There are two varieties of the slate, only one of which is of value (the grey-blue), the other is rejected and thrown on the rubble tip. Brown rusty oval spots mark the slate, and occa- sionally white spots also characterise it. The brown spots are seldom met with in the grey-green slate, while the white are practically confined to it. Fossils are common in certain bands, but are nearly all the well-known Spirifer verneuili (Murch.). Curious concretions of pyrites which the quarrymen call snakes and fish, are occasionally seen.^ These well-cleaved slates merge laterally by insensible grada- tions into a dark-grey highly micaceous schistose rock, well seen at Slaughterbridge. At Villaparks this rock is garnetiferous, as figured on Plate IV. Eastward the schist becomes more mica- ceous until, near Camelford, it is a typical mica-schist spotted with large sheaves of andalusite. II. The Woolgarden Phyllites. — Perhaps the most con- spicuously marked member of the series is the Woolgarden phyllite. It takes its name from Woolgarden Farm, which is situated near the river Inny, in the north-eastern corner of the map. Here the phyllites are typically developed and admirably exposed. There are two varieties of the rock, but both, on cross fracture, show banding, due to alternating laminae of quartz and chloritic or sericitic minerals. The commoner form is silver- grey in colour and possesses a saccharoidal texture; the other is grey-blue and has a well-developed cleavage. The surfaces of fragments are dotted over with small lenticular crystals of a chloritoid mineral, which is described on p. 32. The phyllite is first seen west of Delabole Quarry, from whence it strikes northwards to the Rocky Valley and Bossiney Bay. This locality marks the position of a change of strike, which now swings towards the south-east. The high downs at Waterpit, Minster, and Hendraburnick are composed of this rock. III. Barras Nose Beds. — In Trebarwith valley, beside a field- path leading to Treknow, a band of white limestone is seen beneath the volcanic rocks. On the adjoining map (322) similar limestone is seen near Trewethet Gut at the base of the lava and jUst above the blue-black slate. In this band Mr. J. Parkinson f oiind crinoid ossicles, and we found shells of brachio- pods (indeterminable). The Barras Nose beds are almost exactly like the Trambley Cove beds, but are not so mueli crushed and brecciated. They are easily traced from Treligga to Treknow Mill ; are well developed near Tintagel, and at Barras Nose in the overthrust mass,^ and at Trewethet, from whence they strike inland as far as Tichbarrow Beacon. From this point eastwards they occur sporadically, but at Trevivian they occupy a fair amount of ground. They seem never to exceed 50 feet in thickness. The junction between these blue-black slates and the lava is marked by the emission of springs, and the presence of these ' The petrography of the slate is given on p. 32, and in the Economic Section a brief account of the quarry and its output will be found (pp. 91-93). * H, Dewey, op, oit. UPPEa DEVONIAN. 31 determines the sites of the farmhouses. The JTinction of the Barras Nose beds and the lava supplies springs for the following farms: — Tregatta, Trethevy, Treforda, Halwell, and Hendra. IV. Volcanic Series. — Above these the great mass of volcanic rock lies. On both sides of the river Inny, in St. Clether and Davidstow, are high, bare crags and bluffs of lava, and on the joint faces of these bluffs pillow-structures are sometimes visible, especially south-east of TreguUand. Usually the rock is highly schistose and composed of actinolite and calcite. It is purple and green in colour, mottled with pink and creamy patches of calcite, and spangled with brown and green mica. Its mode of occurrence in the field is very characteristic. As the rock yields an excellent soil for grass-growing it is usually covered with rich permanent pasture, but here and there large craggy masses protrude through the turf, showing weathered edges of the schistose bands. Occasionally, however, there are harder masses which in some cases are cores of epidiorite, probably later than the lava, in others, unsheared lenticles of the lava itself, which are porphyritic; these are especially well exposed in the crags above the Inny, south of Treglasta. In a quarry near David- stow Post Office, pillow-forms are preserved. The vesicles are so large and full of calcite that the rock was quarried here and burnt for lime, but it ran to a slag, and the practice was dis- continued. At Tregrylls, in Sheet 322, the same thing happened. Pillow-forms, rich in calcite, are seen at Trebarwith Strand, just below the old hoist on the foreshore. To these lavas the picturesque scenery of Trebarwith valley is due. The lavas are often rich in magnetite, as at Treknow and Barras Nose, and haematite, near Tamill, in the north-east corner of the map. Y. Tramhley Cove Beds. — These slates are always blue-black in colour, very hard and splintery, siliceous and often brecciated. Excellent sections of them occur near Treligga, at Trebarwith, and in the north-east corner of the map at Treglasta, Abbott's Hendra, and TreguUand. They probably do not exceed 40 feet in thickness. Lines of springs often mark the junction with the lava, and on Sheet 322 the following farmsteads are built at the junction of the volcanic rocks and the Tramhley Cove beds : — Tredole, Tregaina, Trewins, Tregatherel, Tregrylls, Gunvillick, Tremblary. VI. The Tredorn Phyllites are grey-green, even-grained, soft rocks, which are seldom suitable for roofing slates. Fossils occur abundantly at certain localities, and are characteristic Upper Devonian forms.* A darker grey variety is common near the coast at Trevalga; it contains abundant crystals of orthoclase felspar, and is veined by quartz and albite. Structure of the Area. — There are numerous exposures of this series which, over the north-eastern district, show a persistent dip to the north-east. The dip varies between 20° and 40° in both sediment and volcanic rock. Near Tre- grylls (Sheet 322) it changes to northerly, and this con- tinues to the coast at Trevalga. Bossiney Bay marks ' See Pattison, Trans. Roy. Geol. Soc. Cormoall, vol. vii, 1865, p. 3 ; Parkinson, Quart. Journ. Geol. Soc, vol. lii, 1903, pp. 408-428. 32 GEOIOGY Op PAD STOW. the position of a great- change of strike. On its northern side the dip is nearly north; on its southern side it is west-north- west, and from this point to Treligga, south of Trebarwith, the dip remains fairly constant. The beds aire thus seen to sweep round in a big elliptic arc. The longer axis of this trends north- north-west and south-south-east, and pitches to the north-west. This pitch of the axis results in the lower beds outcropping from beneath the higher as we proceed towards the south-east. The whole area, therefore, is a denuded anticline. The hemidome terminating this anticline has been eroded by the sea at Bossiney Bay, so that the lower beds are there exposed ; while to the south of the hemidome subsidiary folds cross the strike. These synclines and anticlines are seen in Trevena Valley. On the west, at Tintagel,^ they are replaced by overthrusts. Petrography of the PJiylUte.s. (la) Slaughterbridge Beds. These beds occur to the north of Camelford and are exposed in the railway cuttings near Camelford Station. They are very dark grey, highly micaceous chloritic phyllites. Near Villaparks they contain garnets full of isotropic grains arranged in patterns such as crosses, chevrons and stars (see Plate IV, fig. 5). The garnets have grown round and included the sericite fibres which can often be traced through the garnet crystals ; but at other times the sericite films are pushed aside and wrap round the garnets. Andalusite is also developed, especially nearer the granite, and scales of secondary white mica with brilliant polarisation tints have grown at all angles to the cleavage. (16) The Delabole Slates. These slates occupy a belt of country from near Delabole northwards towards Waterpit Downs, and an overthrust mass at Tintagel Cliffs. They are of two varieties which graduate one into another ; one being grey-green in colour and having poor cleavage, is rejected in the process of manufacture ; and the other, workable, variety, is grey-blue, very fissile, resonant and of a close and hard texture, and is strong and elastic. Its lustre is silky to resinous and thin flakes are translucent at the edges. The slate is f ossilif erous with the fossils preserved in chlorite. Under the microscope the rock is seen to be a typical sericite phyllite with some chlorite and numerous secondary white micas, rutile in rods, grains, and beautiful networks. There are also small garnets and zircons. Small prisms of tourmaline occur scattered through the slide, with quartz grains and obscure almost isotropic granular matter. The slate has been thoroughly altered and is in no sense a clay slate. The strain- slip cleavage so common in the grey slates further south is nowhere seen in the Delabole slates. (II) Woolgarden Phyllites. In hand specimens these phyllites are silver- grey in colour, flecked with small spindle-shaped green crystals. On cross fracture the rock is seen to be banded owing to alternating laminae of greenish chloritoid and silvery sericite. In some localities the rocks are rich in quartz, as near Trewassa and Newhouse. Cleavage is poor and the ptyllite is not quarried for roofing slate but supplies good building stone (see Plate IV, fig. 6). Under the microscope it is seen to consist of felted laminae of sericitic mica feathering out and partially over- lapping each other but lying flat in the planes of cleavage. Obscure material of low birefringence occurs between the laminae ; niuch of this material is probably quartz. Strands of chloritic materials traverse certain parts of the slide, and with these are associated well developed crystals of chloritoid with ragged edges. The rooks are characterised by the presence of abundant tabular white micas developed later than -and without regard to the cleavage. These micas are strongly birefringent and often occur intergrown with the chloritoid on each side of it. Minute rod-shaped and granular inclusions occur in the tabular micas. Ilmenite and sagenitic rutile also occur with clastic tourmaline. The mineral of greatest interest in these phyllites is the chloritoid which occurs in twinned crystals with ragged terminations and frequently shows hotir- glass structure. Its physical and optical properties shew it to be different from ' H. Dewey, op. cit. tJtPER: DEVONIAN. 33 any mineral of this group hitherto described. The following are the characters : hardness, about 4 : specific gravity, 2-956 ; mean refractive index, 1-630 : bire- fringence a little lower than that of quartz in same slide : pleochroism, for rays a,t right angles to the basal plane, olive green ; for rays parallel to same, yellowish green, optical character — : elongation +. The mineral is not gelatinised by acid (HCl). Its low specific gravity relates it to the chlorites, but its hardness, refractive indices and resistance to acid shew its position to be among the chloritoids. At Tintagel it is replaced by ottrelite.' (Ill & V) The Trambley Cove and Barras Nose Beds, or upper and lower Blue-black Slates. As their name implies these slates are blue-black in colour and are easily recognised by this character wherever they occur. They follow the Woolgarden beds from east to west above and below the volcanic rocks which lie between them. They are admirably exposed and may easily be studied at Trebarwith Strand. In thin sections they are seen to be almost wholly siliceous, very fine- grained quartzites, permeated through and through with fine carbonaceous matter. Usually they have suffered brecciation and subsequent cementation by quartz. In some places near Tintagel they are largely built up of numerous long prisms of tourmaline, but when such varieties are seen they are always beneath the volcanic rocks. (VI) The Tredorn Phyllites. These are the uppermost beds found beneath the Culm rocks in this district. They are sericitic grey phyllites but vary slightly in colour to darker grey. Near the coast at Trevalga (on the next map, sheet 322) small white felspars are scattered through the rock. These are considered by Mr. Parkinson to be orthoclase.' Analyses of Devonian Slates and Phyllites. I. n. III. IV. V. SiOj 55-44 58-30 57-76 51-9 66-6 TiO, 1-20 -23 •90 -8 •8 A1,0, 22-23 21-89 20-62 21-5 19-6 Fe,03 -92 7-05 1-85 -8 -9 PeO 6-83 2-57 6-08 5-1 -2 MnO •37 — -15 — — (CoNi)O -06 — — — — BaO nt. fd. — — — — CaO •33 -39 -40 31 •4 MgO 3-55 1-09 2-41 25 •3 KsO 4-08 2-45 3-10 33 •7 Na,0 1-68 1-18 1-46 -3 9-8 H2O (105° C.) ... HjO (above 105° C.) -08 3-17 } 4-61 5-03 I 5-5 -8 P,0, -23 — ■16 — — FeSj nt. fd. -11 -6 — CO, nt. fd. — — 3-1 — Total 100-06 99-76 100-03 99-1 100-1 I. Banded serioite-chloritoid phyllite, Trewassa, North Cornwall (anal, in Survey laboratory by E. G. Eadley, B 6532).' II. Grey-blue roofing slate, Delabole, Cornwall, mean of two analyses by J. A. Phillips, Land. Edin.and Dull. Phil. Mag., 4th Ser., vol. xli, 1871, pp. 95 and 96. in. Grey-blue Delabole, Cornwall, anal, by B. Blount, F.I.C., published by Old Delabole Slate Company. IV. Shale, Trevone Bay, near Padstow, anal. J. H. Player in H. Fox, Trans. Roy. Oeol. Soc. Cornwall, vol. xi, 1895, p. 723. V. Adinole, Dinas Head, anal. J. H. Player, in H. Fox, Geol. Mag., 1895, p. 19. > W. M. Hutohings, Geol. Mag., 1889, pp. 214-220. ' Quart. Journ. Geol. Soc., vol. lix, 1903, pp. 408-428. ' These figures refer to specimens in the Survey collection. 34 GEOIOGT OF EADSTOW. In the foregoing pages -we have descrihed separately the areas oi unaltered and altered Tipper Devonian rocks, and have pointed out as far as possible the structure and lithology of each. It may be useful now to suggest the correlation which seems most probable, for when we come to compare the two areas we discover a parallelism in the sequence which can scarcely be the result of accident : — Unaltered. Altered. VI. Purple and pale-green slates = VI. Tredorn Phjllites. V. Dark slate (very thin) ... = V. Trambley Cove Beds. IV. Pillow-lava ... ... = IV. Volcanic Series. III. Blackslateof Daymer Bay, = III. Barras Nose Beds. &c., with limestone bands. II. Striped slate with Tenta- = II. Woolgarden PhyUites. culites. I. Slate of Camel Quarry ... = I. Delabole Slates. This correlation, however, is put forward as merely tentative, as the evidence is not sufficiently clear to enable us to form a definite opinion. One difficulty, amongst others, is that the pale slates with thin bands of grit, forming the highest part of the purple and green slate at Dinnabroad, are not recognisable in the Tredorn Phyllite, though, of course, such grits would remain almost unaltered. It is possible, however, that these gritty beds may represent sandy strata higher in the sequence and equivalent to some of the sandy strata full of C'lymenia associated with the Petherwin Beds near South Petherwin, a few miles east of the district described in this Memoir. On this hypothesis it is evident that the Culm Measures, shown on the map on p. 29, must overlap or be thrust over the Devonian rocks, so as to hide the. upper part of that series. 3r5 CHAPTEE IV. DEVONIAN VOLCANIC EOCKS AND GEEENSTONES. LiTBBATUEE. The igneous rocks of the district have be>en described by many authors during the past seventy years ; but the descriptions relate chiefly to field relationships and macroscopic characters. Atten- tion, however, may be called to one or two important remarks made by the earlier writers. The submarine character of the lavas was noted by the Eev. David Williams* in 1841, and their pillowy structure was described by Nicholas Whitley in 1848^ as resembling ' bales of cloth.' Sir H. De la Beche^ differentiated the lavas and tuffs from the sills, but was in doubt as to the origin of the volcanic rocks of the northern parts of the district. Other early papers relating to the igneous rocks are mentioned in the footnotes below.* John Arthur Phillips' was the first observer to attack the petrography in a modern manner by means of microscopic exami- nation of thin slices of the rocks, chemical analyses, and careful observation of their mode of occurrence in the field. In this way he dealt with the roofing slates of Delabole and the lavas of Trelill and Bokelly. In 1878 he described very fully the igneous rocks in his paper ' On the so-called greenstones of Central and Eastern Cornwall.'' In this paper an advance in the direction of classification was made; the first since De la Beche roughly outlined the three groups of igneous rocks. The sills and lavas of St. Minver were described by F. Eutley, and later by Dr. Teall.' Mr. Maynard Hutchings,' in two papers written in 1889, deals with the volcanic rocks and associated Upper Devonian sediments (phyllites) lying north of Bodmin Moor. In these he described the occurrence of ottrelite in the sericite phyllites. This was the first time the mineral had been detected in Britain. The series of papers describing the geology of the coast of North Cornwall, written by Mr. Howard Fox," contain > Rept. Brit. 4 ««oc., 1841, Trans. Sect., pp., 61-62. ' 30th Ann. Rept. Roy. Inst. Cornwall, App. vi, p. 62. ' ' Geological Eept. Cornwall, Devon, and West Somerset,' 1839, pp. 58, 88. ' Sir H. Davy, Trans. Roy. Geol. Soc. Cornwall, vol. i, 1818, p. 46. H. S. Boase, Ibid., vol. iv, 1832, p. 166. Eev. D. Williams, Ibid., vol. vi, 1846, p. 122. S. R. Pattison, Mth Ann. Rept. Roy. Geol. Soc. Cornwall, 1847, p. 7. " H. B. Holl, Quart. Journ. Geol. Soc, vol. xxiv, 1868, pp. 418-420. ' QiMrt. Journ. Geol. Soc, vol. xxxi, 1875, p. 319. 8 Ibid., vol. xxxiv, 1878, p. 471. ' Ibid., vol. xlii, 1886, p. 392. ' 8 ' British Petrography,' 1888, pp. 228-232. 9 Geol. Mag., 1889, pp. 53, 101 and 214. "> Trans. Roy. Geol. Soc. Cornwall, xaX. xi, 1895, pp. 687-724; vol. xii, part viii, 1903, pp. 649-682, and vol. xiii, part 1, 1905, p. 33. 36 GEOLOGY OF PADSTOW. valuable petrographical work on the igneous rocks and the adinoles. Some of the descriptions were supplied by Mr. J. Parkinson and Dr. Flett. Mr. J. Parkinson^ also contributed an excellent paper ' On the Geology and Petrography of the David- In 1907 and 1908, Reid and Dewey" described the strati- graphical relations of these rocks, and also discussed the origin of the pillow-lavas of Port Isaac and district ; and in 1909 Dewey^ described the petrology of Tintagel. Volcanic Rocks. In the last Chapter allusion was made to various indications of contemporaneous volcanic activity during Upper Devonian times. The Lower Devonian rocks of this area yield no clear indications of such activity, nor do the Middle Devonian, unless the lowest band of tuff, here taken as the base of the Upper Devonian, belongs in reality to a slightly older period. This basal tuff has not been recognised on the coast, but as it is often very thin it may easily have been overlooked in the vertical cliffs between Porthcothan and Treyarnon. It is first seen two miles inland, close to Higher Trevorgus, as a thin band striking east and west. Its outcrop, however, is immediately shifted half a mile to the north by a fault, reappearing within a quarter of a mile south of St. Merryn Church, where the tuff forms a conspicuous conical mound in the level fields. About half-a-mile further east, near the high-road to Tresallyn, there is a quarry in a thick mass of this tuff. It is a banded grey rock of varying coarseness, composed of coarsely crystalline material with many white felspar crystals; at this spot it con- tains but little shale, and we did not observe in it any other fragments of sedimentary rocks; we could find no fossils. Another smaller quarry has been opened close to Trevisker, where the band seems to be shifted by an oblique fault in such a way as apparently to divide into two branches. From Trevisker the tuff can be traced eastward as far as the Padstow and Newquay road. Then there is a gap of about a mile, but it is picked up again along the same strike within a quarter of a mile of Trevilgus, seemingly much reduced in thickness r In the s-teep bank of the Bodellick creek the tuff is well seen, and is only three feet thick; though it is not clear whether this thinning is original or the result of shearing. East of the creek the band again thickens, but the tuff becomes finer grained ; two quarries in this finer tuff will be found south- east of Bodellick, but the thickness of the band cannot be measured. Eastward it again thins, and though it can be traced almost continuously to the high-road at Dunveth, in the road- cutting it is only ten inches thick. > Quart. Journ. Geol. Soc, vol. lix, 1903, pp. 408-428. ' Ibid., vol. Ixiv, 1908, pp. 264-272 ; and ' Summary of Progress ' for 1906 (Mem. Geol. Surv.), pp. 38, 39, and for 1907, pp. 38, 39. . » Qmrt. Journ. Geol. Soc, vol. Ixv, 1909, pp. 265-280. , VOLCANIC aocKS. 37 From this point it becomes extremely difficult to trace so thin a band, especially as the strike changes rapidly and the whole country is obviously much disturbed. Tuff reappears, however, about a mile west of Wenfordbridge. Though this band of tuff was probably deposited in the sea, it has thus far yielded no fossils. It is essentially composed of coarsely-felspathic, igneous material, with comparatively little vesicular rock and no large quantity of mud; broken up sedi- mentary material occurs rarely, and usually in small fragments. No lava flow has yet been noticed on this horizon, and neither pillows nor volcanic bombs seem to occur. If we turn next to the difficult area of Dennis Hill, Ball Hill, and Cant Hill, we find porphyritic tuff so similar to that just described that it may well belong to the same eruption. In the Camel cliff north of Dennis Cove, there is a 5-foot band of this tuff, with a>thinner band some feet below. Dennis Hill is capped by tuff (formerly quarried), and seems to be encircled by a lower band, 15 feet thick, and well seen in the cliff near Sunnycoruer. A detached mass of this tuff caps Ball Hill. The north side of the Camel exhibits a greatly increased development of tuff. Cant Hill rises to 247 feet above Ordnance datum, is flat-topped and some 450 yards long. Practically the whole of the upper part and much of the southern slope is com- posed of pyroclastic rock of the type described by Rosenbusch and others as ' schalstein.' The tuff, however, is probably much less thick than would appear, for it has a sharp dip towards the river. Whether the tuff of this particular area is really the basal tuff is doubtful. It is at least as probable that it is a continuation of the lava next to be described. The abundant fossils in the slate below supports this view, though the species are not sufficiently characteristic to decide the point. For a considerable height above the basal tuff there is no sign of renewed volcanic activity, but in the grey slates, a short distance below the purple and green strata, we meet with a thick and extensive sheet of lava of very peculiar character. We have not been able to discover the actual vents from which this lava was extruded, but as it thickens in two different directions and almost disappears between, it probably flowed from two vents in almost contemporaneous eruption. We have elsewhere' attempted to account for the origin of the singular ' pillow- structure ' exhibited by this lava near Port Isaac ; but there are various other points of interest which could not then be dealt with. The maximum thickness of lava is found in the cliffs west of Port Isaac and in the inland crags north-east of Camelford. Between these two areas the sheet becomes very thin, a few miles to the south-west, towards Padstow, it dies out, though a trace of it is still found in the cliff west of Padstow, near Porthmissen. It will, therefore, be convenient to commence with the best sections of the thick mass, for towards its margin the structure is difficult to interpret, unless the undisturbed thicker sheet has ' Eeid and Dewey, 'The Origin of the Pillow-lava near Port Isaac in Cornwall.' Quart. Joum. Geol. Soc, vol. Ixiv, 1908, pp. 264-272. 38 GEOLOGY OP PADSTOW. first been examined. The lava north-east of Camelford is much sheared and highly altered; its petrology will' be dealt with in this Chapter ; its position and probable stratigraphical relations have already been referred to (see ante pp. 31, 34). It will be seen on reference to the map that lava occupies a considerable area between Pentire and St. Kew. It is first seen in the bold clifi of Pentire, where it attains its maximum develop- ment, the cliff, entirely lava, having a height of 258 feet. The total thickness may be much greater, but, on the other hand, there may be some duplication by overthrusting ; great masses, however, are quite free- from shear-planes. The lava is highly vesicular, and the mass consists of an accu- mulation of detached pillows piled one upon another. The pillows seem to have rolled over and over,, and to have settled down while still sufficiently plastic to take impressions of the contiguous spheroids. They cannot, however, have been very fluid, for triangular spaces were com m only left where adjacent spheroids did not fit. These interstices and the vesicles were afterwards filled with nearly pure calcite, which in some places has been replaced by chert. The pillows vary in size from about 1 to 8 feet in diameter, the majority measuring, perhaps, 2 or 3 feet. Each has definite boundaries; and, even where the surfaces are in close contact, they are separated by a thin skin from the adjacent spheroids. The internal structure of the pillows is shown in the sections exposed on joint-faces, and each is seen to consist of a series of alternating concentric shells of more and less amygdaloidal rock. The amygdules are smallest in the outermost layers, but increase greatly in size towards the interior. The centre is highly vesicular, often with a large cavity from which the in- filling calcite has been weathered out. Some of these central cavities occupy a large area of the section, measuring as much as 2 feet across in a pillow 6 feet in diameter. In short, each pillow is a gigantic thick- walled bubble, blown out by the in- cluded steam and other gases, and showing the stretching of the walls in the concentric bands of drawn-out vesicles which occupy the outer part of its substance.^ The exceptionally spongy nature of this lava may help to explain the formation of the pillows. Each pillow was a bubble, so light that, while hot, with the cavities full of gas, and the whole surrounded by a shell of steam, it nearly floated. The bubble probably spun round and round in the water till it began to cool and form a skin, then it settled down, but was still suffi- ciently plastic partly to mould itself to the adjacent pillows. On the coast south of Pentire Point we find cliffs formed of grey striped slate for half a mile north of Pentire Haven ; then interfolded bands of slate and lava make their appearance. The sudden increase in height of the cliff, and its sheer, almost perpendicular, face, mark the commencement of the great development of lava, and from this point to the first bank of Cliff Castle the whole cliff consists of lava. Nowhere is the spheroidal structure better seen than on the west side of the ^ Photos 455-465, 523, 524 in the G-eoIogical Survey collection of BngKsli photographs illustrate this pillow-strncture. VOLCANIC ROCKS. 39 neck south of Cliff Castle. If the lower pa,th is taken, a point will be reached where a mass of lava has fallen away, leaving a smooth, vertical, joint-face. For a length of over 30 yards and a height of 20 feet an accumulation of large spheroids of vesi- cular lava, often separated by partings of shale and calcite, is exposed. A little further north interbedded lava and striped slate is again met with. Kellan Head also shows excellent sections of this lava, the best being near the top of the cliff, close to the Head. It is reached by descending the steep grassy slope and turning sharply to the left, the track leading to a narrow shelf immediately below an extensive joint-face. At this point it is noticeable that the in- filling of the vesicles and of the spaces between the pillows is entirely chert, which has replaced the calcite. This chert often looks very like the Devonian radiolarian-chert. Another good section will be found at the foot of Eeedy Cliff, but is only accessible by descending a very steep grassy slope and then climbing over fallen rocks. The lava here is let down between two faults, and another of these north-north-west faults cuts off the mass of Kellan Head on its east side. Scarnor Point again shows a cliff of lava; but then the outcrops run inland. Of the inland sections we would especially recommend for examination the Church-hill road-stone quarry at Port Isaac. This will be found close to the high-road just outside the village, but it is not seen from the road. A quarry by the roadside shows the lava, but not particularly well ; immediately above it, entered by a gate and cart-track a little higher up the hill, is a newer quarry, where the large hollow pillows are almost as well seen as on the coast. Certain of them have been decalcified, and the large central cavities now contain only a handful or twr of loose ' wad ' or manganese oxide. The small pillows, polishes and placed in the Museum of the Geological Survey, came from this quarry. It is worthy of note, that whilst in the upper of these contiguous quarries the vesicles and interspaces are filled with calcite, in the lower the calcite has been replaced by chert. Another excellent deep section was seen in a quarry for walling-stone on the south side of the road to Portquin, and a quarter of a mile north of Trelights; but before we left, the lower half of this quarry had been filled up . Thepillows at this spot seem to a large extent to have broken up while hot into clinkers, which have tended to coalesce into a mass of coarse breccia; specimens of this curious rock have been placed in the Museum. Other similar sections will be found in St. Mabyn and St. Tudy. Fragmental rocks of this character usually occur towards the thin edge of the sheet, away from the centre of eruption, and they seem occasionally to graduate into true tuffs, though on this horizon tuff is rare, unless, as is not improbable, the tuffs of Cant Hill, Ball Hill, and Dennis are of the same date. The curious section at Burniere has already been described (p. 23). At Sladesbridge also the tuff yielded numerous fossils, including a doubtful Clymenia. A small quarry, immediately west of Trelights, shows a good section of the pillows, and one close to Pennant exposes the junction of the lava and slate. Midway between these sections 40 GEOLOGY OF PADSTOW. and the Church-hill quarry at Port Isaac comes, however, a narrow band of tuff. Whether this is on the same horizon as the lava is not clear; sections will be founds in a quarry just below Trewetha and in another on the opposite side of the valley, above the mill-pond. It will be observed that south and east of Port Isaac a number of thin bands of lava have been mapped. It is impossible to make sure whether these are merely the faulted and folded out- crops of the same bed, becoming thinner in, a north-easterly direction; but the field evidence seems in most cases to favour this view. The mode of occurrence of the smaller outcrops is well shown in the cutting just outside Port Isaac, on the road to the station, and also in the high-road near the Headlands at Portgaverne. At Treore, however, there is a thin band of tuff higher in the series, in purple and green slate, and what is probably the same volcanic band reappears as a thin vesicular lava further to the north-east, near Dinnabroad, and in the cliff below. The exact relation of the volcanic outbursts to the colour- boundary in the slates is a puzzling question. Great part of the district is under grass or is only occasionally ploughed. If we could watch it for a series of years, till the whole in succes- sion had been ploughed, we might be able to give some satisfactory explanation of the curious way in which the colour- boundaries behave between St. Endellion and St. Kew; but for this we cannot spare the time. We can only say that near Treharrock, for instance, a bed of vesicular lava seems to cut across the colour-boundary between the grey and the purple and green slate, or, perhaps, rather, that the colour-boundary cuts across the lava. At any rate, the lava at many places is in contact with purple and green slate, though more often the lava is found some way down in the striped grey slate. Petrography. The volcanic rocks are spilites, schalsteins, and tuffs. In hand specimens they are for the most part grey-green in colour and conspicuously vesicular. Weathered fragments are often pumiceous owing to the solution of the calcite from the vesicles Although they constitute most of the highest land in St. Minver, St. Endellion, St. Mabyn and St. Tudy, they seldom form baro crags, except near Lanow, where high craggy rocks compose the Wastrel. Most of the lavas are vesicular ; a few are porphyritic, and the tufEs contain vesicular fragments, but it is difficult to say in many oases whether the rock is a sheared lava or a tuff. The vesicles are lined with chlorite and filled in with crystalline calcite (Plate iv, fig. 4). In other cases quartz and calcite occupy the vesicles, and each of these minerals at times fills the cavities to the exclusion of the others. In some the chlorite lining is confined to one side of the vesicle, the rest being filled with calcite. The Bokelly lava is distinctly por- phyritic with large idiomorphic felspars, probably albite. The rock at Cant Hill is also porphyritic and contains much calcite ; it is a typical schalstein. Under the microscope the slides present a great similarity of structure, but while some of the rocks are glassy others (Plate iv, fig. 4) contain a devitrified groundmass full of lath-shaped albite felspars surrounding numerous vesicles. The ferromagnesian constituents are replaced by chlorite and were probably chiefly granular augite. These rocks are very closely related to those of the Plymouth district described by Dr. Plett.' • In ' Geology of Pl^outh and Liskeard ' {Mem. Qeol. Surv.), 1907, p, 94, VOLCANIC aOCKS. Analyses of Devonian pillow-lavas. 41 I. II. III. IV. V. VI. SiO, 43-23 40-05 36-74 48^66 49-90 40-55 TiO, — — • — .,- 2-95 A1,0, 21-37 20-46 17-55 2327 21-09 16-65 FejOa 1-69 1-83 1-62 •77 -72 101 PeO 9-53 12-66 8-23 13^12 11-93 9-46 MnO trace trace trace trace trace -20 (CoNi)O ... — — — — — -07 OaO 6-66 6-62 12-08 carbonates deducted. -68 6-06 MgO 3-57 4-28 6-04 3-20 6-86 5-20 KjO trace trace •74 -57 •63 ■27 Na,0 5-63 4-82 3-08 517 4-34 4^76 H ■[ hygrometrio " \ combined . -51 -56 •38 -92 -70 •27 3-98 4-30 3-93 4-38 3-10 3^89 P.Os -97 -63 •35 trace trace •73 CO, 2-61 3-67 9^10 carbonates deducted. carbonates deducted. 7^85 PeS, -33 trace — — — •18 100-08 99-88 99^84 100-06 99-95 100-10 Sp. gr. 2-82 2-83 2-82 2-80 2-78 Proportion of Car- 25-73 1201 bonates to rock. I. Lava from Pentire Point. II. Left of the main road to Port Isaac, on I the south of village. i in. Lava, Bokelly, St. Kew. ' IV. Vokan^cjock, TreliU Quarry, |j. ^ pi^aiips, Quart. Journ.Geol. Soc, I J. A. Phillips, Quart. Journ. - Geol. Soc, vol. xxxiv, 1878, pp. 482, 484. V. do. i mile south of IV. I ' ^°»- ^-^^-"1875, pp. 326-328. VI. Porphyritio spilite, Devonport Workhouse Quarry (anal. E. G. Eadley), Geology of the Coun-try around Plymouth, Mem. Geol. Surv., 1907, p. 97 [E. 4947]. The Volcanic rocks of the northern area form a belt of country nearly a mile wide traversing the parishes of St. Clether, David- stow, and Minster in a north-westerly direction. For the greater part of this distance the lavas are in a schistose condition, but at several places pillowy forms are preserved. Chlorite and actinolite schists rich in calcite are the inost usual forms in which the rocks occur, but vesicular and porphyritic varieties are found lying among the more sheared lavas. There are also several bands of haematite, formerly worked, occurring in association with the volcanic rocks, which appeiar to be due to replacement of the lava by iron ore. The vesicular character of the lava is often preserved in a rock which is essentially hsematite. Other belts and bands are rich in magnetite, but these are too small to be profitably worked as ore. Under crossed nicols these rocks are very beautiful objects owing to the brilliant polarisation tints of the epidote contrasting with the paler hues of the felspars and actinolite. Their sehistose character is rendered evident by bands of chlorite 43 GEOLOGY OF PADSTOW. or actinolite alternating with calcite and felspars. The augite is rarely fresh, being usually replaced by actinolite and chlorite. Biotite and green mica are the other ferro- magnesian constituents. The felspar is usually albite, but is possibly secondary, as there is a great quantity of epidote in well- formed prisms and plates which in places enclose allanite. Near Tintagel large felspars are found in the lavas which also contain much secondary quartz. The rocks from the following localities strongly resemble one another : — ^the lava from the lane leading from Trevena to the valley, the rock at the base of the cliff below the Castle Keep and the lava composing- the little headland on which the boat is suspended in Tintagel Haven. Greenstones of the Sottthern Area. 1. These are divisible into three well-marked types: — (A) dia- bases; (B) proterobases ; (C) quartz-diabases. (A) Diabases. These rocks occur in the field as sills, or more rarely dykes, mainly intruded into the lower beds of the Upper Devonian. The sills vary in thickness from 5 up to 120 feet, and at Stepper Point a thick dyke is seen. In the hand specimen they are dull grey-green in colour and are typically non-porphyritic. Vesicular types occur by Pentireglaze and at Trebetherick, but these are exceptional. In texture they vary from fine-grained rocks of uniform colour to coarser-grained varieties with a mottled appear- ance due to grey or lilac patches richer in felspar being scattered through a dark green matrix. The fine-grained rocks are mostly developed at inland localities and these sills are not so thick as those of the coastal regions. Large sills show difference of texture in the centre and at the margins. Under the microscope the two varieties differ as will be described. In both varieties olivine was probably present in some cases, but is now represented by serpentine and chlorite, and is never fresh. Augite is the chief ferromagnesian constituent. It occurs as large plates, enclosing lath-shaped felspars in typical ophitic manner and is often purplish in colour. It rarely shows pleochroism and usually possesses an extinction angle of 45°. Chlorite replaces it in many of the slides. The augite also occurs as anhedral crystals of the same age as the felspars (Plate IV, fig. 2). No other ferromagnesian mineral is found in any of these rocks. The felspars vary in size from large platy crystals to small rod- and lath-shaped microliths. This difference of size depends upon the relative period of crystallisation, the small ones being of early growth while the larger crystals were developed pari passu with the other rock constituents. Usually the felspar is rich in soda and it is normally albite. The small crystals possess the same optical properties as the large ones. Apatite is abundant and occurs as large prisms with dark cores. Some crystals are over 2 mms. long. Ilmenite and titaniferous magnetite occur as grains and skeleton crystals often weathered to leucoxeno. The diabases from the following localities, all of which are near the coast, are characterised by the presence of large platy felspars : — Stepper Point, Mouls Island, ClifE Castle, Pentire and the outlying islets of Newland and Gulland ; Lower Trevarra and St. Michael's Church, also at Kitt Hill. The coastal sills are distinctly less ophitic than those of the inland localities, but their margins are more ophitic and finer grained than their centres. AH the inland sills possess numerous small rod-shaped felspars and have a well developed ophitic structure. Many of these sills are fresh, as at Croan House, Kestle Quarry and Sladesbridge.- Similar rocks oeciir in a state of decomposition at Egloshayle, GKEENSTONES. 43 Bali; Treworder Wood and' Outtingj North Trevarner, and the Town Quarry, Wadebridge; Two of these rooks were analysed by J. A. Phillips. I. n. SiO, ... ..J AljO, ... FejOs ... ;.. PeO ... ;.. GaO ... Mga- KjO; Na,0 :■ ... tt'q ( hygrometric ' ( conlbined PiOs ... CO, S03 38-53 46-65 15-41 19-36 2-66 2-61 13-66 9-55 8-95 4-08 8-66 5-34 ^47 -32 2-61 4-54 ■45 -40 • 4-71 4-96 ,-37 trace 3-21 1-90 trace — 9969 99-71 Sp. &r. ... ... ... 2-88 2-78 I. Dark-green trap interbedded with vesicular lavas at Pentire Point. J. A. Phillips, Quart. Journ. Geol. Soc, vol. xxxiv, 1878, p. 477. II, At Sladesbridge, three-quarters of a mile east" of Bgloshayle Church, op. cit., ■ : p. 478.. One diabase-mass is of such exceptional cliaracter as to deserve special mention. At Stepper Point a wide dyke rises nearly ver- tically through the purple and green slates (Plate ll.), cutting across the bedding and causing great alteration and spotting for a considerable distance on either side. The amount of alteration caused by this mass, and the absence of any chilled margin, make it probable that this dyke fills a passage through which the molten rock flowed for a considerable time ; it may represent a pipe of Carboniferous age. It is singular that the slate in contact with the igneous rock shows no cleavage independent of the bedding. The metamorphism caused by this dyke is more fully described on pp. 49, 50. (B) Proterohases . These rocks are allied to the diabases but are typically non- ophitic and very rich in brown hornblende and biotite. They are restricted to two Avell-defined areas. One of these is near the. coast. From Cataclews Point and Trevone Bay a band of this rock runs to the left bank of the river Camel at St. Saviour's Cove and reappears on the right bank of the river near Rock and at Cassock Hill. Another band is first met with at Bray Hill, .traverses the parish of St. Minver, and forms the high ridge on which the village of St. Minver stands. The second area is inland and lies some miles to the east, in the parish of St. Mabyn. Two wide belts are seen to the north and east of tjie village, while a small patch is met with between the village and the river Allen, and another near Lemail Farm. Many of the proterohases have once contained olivine now converted into serpentine. Soda-lime felspar is not visible in the slides, and if it were originally present is completely decomposed. They contain, however, abundant alkali felspar, and this is one of their most distinguishing features, giving them a special interest. It occurs in many of these rocks as large plates, enclosing idiomorphic augite and hornblende (Plate lY, fig. 1). Occasionally it shows polysynthetic structures with symmetrical extinction angles about 12° and positive optical sign, and for all rays has a lower refractive index than the Canada balsam. These properties show that it is 15999 D 4^ GEOLOGY OF PADSTOW. albite ^possibly with albite-oligoclase). On the other hand some broad plates are often untwinned or snow only an evanescent twinning which is of a cross- hatched type, and these felspars are soda-microcline and anorthoclase. Apatite occurs in numerous large hexagonal prisms, , Similar rocks from the Plymouth area have been described by Or. Plett,' [who has ref^ryed them to the pro- terobases, and this name will be adopted provisionally here. The olivine is always enclosed in augite or , hornblende ; it is completely decomposed into serpentine and is recogijisable only by its shape. The angite has generally a marked purple-brown colour and a fairly strong dispersion of the axes. It occurs in large and small crystals, the. lattei; being more idiomorphio and not enveloped in hornblende ; hour-glass structure , and slight pleochroism are also present. The large crystals of augite are surrounded by or intergrown with hornblende in perthitic fashion (Plate IV, fig. 1), each of the minerals forming about half of these aggregates. The ho.rpblen^e is generally later in crystallization and less idiomorphio than the augi.te, and, when the two minerals are intergrown the. amphibole is peripheral ; many of its crystals however contain no augite. From its colour and pleochroism it seems to correspond to the barkevicite of Rosenbusch. In some cases the hornblende encloses small crystals of decomposed felspar in an ophitic manner near its margins. Biotite is quite common in many of these rocks, never 'idiomorphio but tabular with irregular edges. It is a little later in crystallization than the hornblende and varies in abundance in difEerent slides of the same rock. Its colours are deep red-brown to clear yellow, and it often shows decomposition into chlorite. Soda lime felspars of basic composition are, as stated, completely absent ; but, certain patches of obscure secondary products may represent them. At any rate they cannot have been abundant. Alkali felspar forms from a third to a half of the rocks. It is the last mineral to crystallise, and its broad plates enclose all the earlier constituents. A few small lath -shaped crystals may occasionally be observed, but the bulk of the felspar is entirely anidiomorphic. The felspar is often very fresh, with its twinning obvious in the sections, and it varies little in composition, being always a plagioclase rich in soda, or a soda orthoclase. Of the accessories, titaniferous iron ores are frequent, occurring as grains or as skeleton crystals which weather to leucoxene or sphene. The abundance of large prismatic apatites is characteristic of the rocks ; they have often dark cores. Secondary serpentine and chlorite are frequent ; carbonates are also common as well as epidote, especially in the inland exposures. The felspars contain flakes of secondary white mica and calcite. Bpidote and chlorite may replace biotite. Pyrites is also a frequent constituent. Quartz and analcite have not been observed in any of these rocks. ," In the hand specimens the rocks bear a strong resemblance to one another. They are granular in texture, and seldom possess porphyritic, and never vesicular structure. In colour they vary from grey-green to deep purplish brown, and those containing much felspar are often speckled with rectangular spots of white and yellow felspar. Biotite-rich varieties are spangled with glistening bronzy flakes (Rock Quarry). Their specific gravity ranges from 2-85 to 2-90. The rock found at Tregaddock, St Mabyn, has a higher specific gravity, it being 3"17 ; but, as will be seen later, this rock also differs in other respects from the typical proterobases. In the field they occur as sills folded with the killas, which at the contact with the igneous rock shows a narrow indurated margin of whitish yellow colour. These sills are seldom more than 70 feet thick, are jointed and weather in spheroidal masses. In Rock Quarry, on the Golf Links opposite Padstow, large spheroids with exfoliating concentric skins are scattered over the floor of the quarry and occur in situ in this mass. The stone at Rock Quarry is fresh, fairly coarse grained, non-porphyritic, of a dark green colour with spangles of bronzy mica, and has a specific gravity of 2'90, (E. 5595 and E. 5596).* Plate IV, fig. 1. Under the microscope this is one of the most typical members of the group. Olivine is represented by serpentinous and chloritic pseudomorphs. The augite is of two sizes, idiomorphio and of a purplish-brown colour, often in perthitic intergrowth with the brown horn- blende (barkevicite). Biotite is fairly abundant. The alkali felspars are fresh and occur in large platy masses enclosing the ferromagnesian minerals The felspar is mostly albite, but soda-microcline also occurs in some quantity.- Titaniferous magnetite or Igucoxene and skeleton crystals of ilmenite are .1 In ' Geology of Plymouth,' (Mem. Geol. Surv.), 1907, p. 98. * These figures refer to rock specimens and microscopic slides preserved in the collection of the Geological Survey. . ,. . . GBiEENSTONES, 4S present with a fair amount of apatite in large prismatic crystals. Mr. Parkinson has described this rock as a doleritic camptonite.' The rook at Bray Hill near St. Enodock Church, is obviously a decomposed proterobase in which leucoxene is abundant. Specimens from the same sill at Trewint Hill Quarry are fresher. Its decomposed felspars are full of epidote and white micas but much albite is present in the rock with augite, brown hornblende and iron ores. The rock was described by J. A. Phillips* as a dolerite. Dr. TealF also mentions it. The rock from Little Cove, St. Merryn, is dark green in the hand specimen and has much hornblende. Its specific gravity is 2-90. It contains a large amount of biotite but all of one generation. A very similar rock but with porphyritic hornblende is found on the north side of Trevone Bay. The augite of two sizes is enclosed in plagioclase felspar for the most part decomposed, with abundant albite in large irregular crystals. The well known Cataclews rock shows under the microscope a rather fine grained holocrystalline aggregate of deep brown hornblende, biotite, serpentinous pseudomorphs after olivine, small augites and decomposed felspars, abundant grains and needles of apa,tite and iron ores. It differs from the majority of rocks in the neighbourhood in its richness in strongly pleochroic biotite. General MaoMahon* described the rock as an altered hornblende picrite, and Dr. Teall^ calls it an ophitic diabase. It shows a considerable variety, there being coarse types (proterobases) and finer grained types resembling lamprophyres. The latter, in which small idiomorphic augites, hornblendes and brown micas lie scattered in a ground- mass of small felspars, have much of the appearance of the camptonites. The rocks of the area around St. Mabyn and Lemail differ from the coastal group in several respects, but chiefly in their occasional ophitic structure and higher specific gravity. The Lemail rock is speckled dark green and white and is fairly fresh. It is found as a sill in a wood near Lemail Farm and has a tendency to spheroidal weathering. Its zoned purplish augite bordered with brown hornblende contains serpentine pseudomorphs after olivine. There are besides typical ophitic patches of hornblende enclosing felspar. Ilmenite, leucoxene, sphene and pyrites, occur in plenty. All the felspars are filled with white mica and other secondary minerals. In other specimens the augite and horn- blende are earlier than the felspar. About a mile to the east of this locality at Tregaddock, St. Mabyn, a rock is quarried in an orchard opposite the farm and can be traced for a considerable distance in the fields to the north and south. It is a reddish brown coarsely crystalline rock with a specific gravity of 3'17. In the slice the rock is uncommonly rich in epidote, but no calcite occurs. Deep brown hornblendes enclose the augites and are themselves enclosed by platy masses of albite felspar veined with decomposition products^ A sample of this rock was analysed in the Survey laboratory by Mr. E. G-. Radley, and the results are given in column 1 of the subjoined tables. Intrusive rocks essentially similar to those under consideration occur in the Devonian rocks of East Cornwall, and have been described by Dr. Flett in the Plymouth Memoir as proterobases. In his ' British Petrography ' Dr. Teall has named them hornblende dolerites, while he reserves the term proterobases for rocks containing augite and secondary hornblende. The members of this group have close affinities with the ophitic diabases of this district, which also are rich in soda felspar ; they occur along with them and exhibit many gradations into the diabases. The chief difference is the scarcity of ophitic structure in the proterobases and the greater abundance of dark hornblende, biotite and inter- stitial alkali felspar. All the minerals named occur in the diabases, only in less quantity. By these peculiarities the rocks of this group are closely linked with the essexites. The purplish augite, dark brown hornblende and biotite and alkali felspar are similar to those of the essexites of Ilongstock on the Elbe and of South Norway, as described by Rosenbusch.' The principal dis- tinguishing points are the greater scarcity of basic soda-lime-felspar (this • Trans. Roy. Geol. Soc. Cornwall, vol. xii, 1903, pp. 676-7. ' Quart. Journ. Geol. Soc, vol. xxxiv, 1878, p. 478. ' ' British Petrography,' 1888, pp. 228-232. * Trans. Roy. Geol. Soc. Cornwall, vol. xi, 1894, p. 699. " ' British Petrography,' p. 232, 1888. ' Rosenbusch, ' Mikroskopische Physiographic,' vol. ii, 1907, p. 403, J 5999 D 2 46 GEOLOGY OF PADSTOW. mineral ia not found in fresh cofadition) in the Cornish rocis and the abundance of albite and anorthoclase instead of orthoolase. Felspathoids such as nepheline and .sodalite have not been found in the proterobases, but as, their oliyine and basic felspars are always decomposed it is not likely, that if these minerals were originally present they would be in a recognisable condition. The chemical analyses of essexites and proterobases given below indicate that the essexites contain more alumina and alkalies, being as a whole more f elspathic in composition. The comparatively high amounts of water in the Cornish rocks show that they are usually to some extent decomposed. They also contain much titanium and more abundant phosphoric acid, iron, magnesia and lime than the essexites. In these, respects they approach closely to the camptonites, as. will be seen by a comparison of the analyses cited. Though they are usually granular there are fine-grained types as at Cataclews, which are panidiomorphic and could almost be. described as camptonites, as was recognized by Mr. Parkinson. Typical camptonites with porphyritic and ocellar structure, rich in carbonates and analcite like those of the Orkneys,' have not yet been met with in Cornwall. It may be noted that both in Norway and Bohemia camptonites accompany essexites. These proterobases from St. Minver do not agree entirely with any described type. They have strong affinities with the proterobases of Gumbel and the essexites of Bosenbusch, but chemically they are related to the camptonites. They are not entirely fresh, and perhaps originally contained basic as well as soda felspars. If this were the case their relegation to the essexites is more justifiable. The presence of large platy albite and soda-microcline felspars gives them a distinctive character, and if they are entitled to a special designation they may be called minverites,' a sub-group of the essexites. Analyses of Proterobases, Essexites and CamptoniteSj I. n. ITT IV. V. SiO,... 42-86 ' 42-88 .47-19 47^94 41-94 TiOj 3-30 3-68 2-09 •20 4-15 AljOs 13-81 14-31 13^96 17^44 15-36 FejOa 1-93 4-70 3^39 6^84 3-27 FeO 11-64 9-44 9-01 6-51 9-89 MnO •44 -32 -47 — •25 (CoNi)O •09 •07 -04 — — BaO nt. fd. nt. fd. •06 — — CaO 12-84 10^90 8'08 7-47 9-47 MgO 7-33 5^67 7-10 202 501 k;,o -13 1^92 •70 2-79 •19 NaaO 2-03 2-52 4-50 5^63 5-15 HjOatlOS'C. ... •13 •25 -12 [ 2-04 3-29 H2O above 105° C... 2^81 2^52 2-56 P.O5... •30 •54 •56 1-04 FeS, •80 •22 •11 — . CO, -22 •13 •79 — 2^47 100-66 10007 100^73 99-92 100^44 Sp, Gr 317 — — — — I. Proterobase, Tregaddock, St. Mabyn, Cornwall. (E. G-. Radley, Anal.) B. 6496. II. Proterobase, from quarry west side of Lynher opposite Grove Landrake. (Anal., B. G. Radley.) Plymouth Memoir, Geol. Surv., p. 100. B. 4763. III. Proterobase, 200 yards north of viaduct, south-west of Cartuther, Cornwall. (Anal., W. Pollard.) Plymouth Memoir, Geol. Surv., p. 100. E. 4770. IV. Essexite, Salem Neck, Salem, Mass., U.S.A., Bosenbusch, Elemente d. Gestein., 3rd edit., 1910, p. 196. V. Camptonite, Campton Falls, N.H., U.S.A., Bosenbusch, op. cit., p. 301. • J. S. Flett, 'The Trap-dykes of the Orkneys,' Tram. Roy. Soc. Edin., vol. xxxix, 1900, p. 887. ' From the parish of St. Minver where they are best studied. GEEENSIONES. 4'f (C) The Quartz Diabases. At Treglennick in St. Merryn a rock occurs which in the hand specimen presents both in colour and texture the usual appearance of a diabase ; but, when examined under the microscope, the struc- ture oi the groundmass is seen to be micropegmatitic. The ferro- magnesian mineral is augite, which occurs in well-formed crystals in places enclosing the lath-shaped felspars. At Park Head a similar rock is found, but in an advanced state of decomposition. Epidiomtes. All the intrusive igneous rocks north of St. Breward are epidiorites. The high-land of Michaelstow Beacon, Tichbarrow (on the map to the north, Sheet 322), the masses by Hendawle, Trevivian, and Higher Basil are all composed of epidiorite. There are considerable differences among the hand specimens, chiefly arising from the size of the actinolite crystals. These differences are emphasized by weathering bringing the actinolite in relief above the groundmass. Sometimes the actinolites are long and narrow; at others, broad platy masses and crystals as much as an inch long. In colour a wide range of varia- tion is found from grey through green to black. They are exceedingly tough rocks and powder under the hammer, and, in consequence of this property, are not worked for road metal. Attempts to quarry by dynamite and other explosives were made but abandoned, owing to the high resisting powers of the rocks and absence of joint planes in them. The high and picturesque crags of Michaelstow and Tichbarrow Beacons are therefore un- scarred by quarries. TTnder the microscope they are seen to possess characters which relate them to the diabases, for ophitic structure is not uncommonly preserved in parts of the slides. Olivine is not found, neither is any augite preserved, but long crystals of green hornblende terminated by divergent sheaves of actinolite occupy much of the slides. This actinolite is strongly pleochroic, varying from very pale greenish yellow to rich deep green. The felspars are often perfectly clear but do not seem to be original. Others have crystalline outlines and are twinned and turbid. Fibres and detached needles of actinolite appear to be imbedded in the clear felspars as though the felspar material had grown around the needles. Pale brown contact micas occur in the Treveighan and Michaelstow rocks, and apatite is a common constituent. Epidote is exceedingly common and is usually pistacite with characteristic pleochroism. Titaniferous magnetite, sometimes decomposed to leucoxene and sphene, is frequent. Unsheared cores of these rocks occur in the midst of the intensely sheared lavas. They are obviously later than the lavas but were intruded before the crustal movements took place which produced the shearing. At the same time good examples of sheared epidiorites occur at Lambrenny (E6535) and (E6502) near Treboy. In the latter rock radial bunches of actinolite needles he in new felspars. A similar rock is seen at Tregue (E6501). The epidiorite of Blisland is so greatly altered by contact with the granite that it will be dealt with more conveniently in the chapter on the Metamorphic Aureole (on p. 76). Sheared Granitic Rock. In Worthyvale farmyard there is a curious sheared igneous rock of a greenish-yellow colour which is worked for building 48 GEOIiOGT OF PAD STOW. stone. This rock appears also- in the railway cutting and m numerous grubbings on the neighbouring moorland, but its rela- tion to the phyllites is difficult to determine. Microscopically its schistose character is evident (Plate IV., Fig. 3). The ferro- magnesian constituents, which are mainly actinolite and biotite, wrap round eyes of felspar and quartz. The felspar shows mul- tiple twinning and strain shadows and is for the most part albite. The quartz is granular and crushed. What the rock _ was originally it is almost impossible to say ; but, of whatever origin, it has suffered shearing with the phyllites, and subsequent meta- morphism. The rock was described by Mr. Parkinson,^ but he was unable to suggest its original nature. Alteration caused by Greenstone. The slates in contact with intrusive sills and dykes of greenstone are always altered, but the nature of the alteration varies accord- ing to the character of the sedimentary rock. Adinoles occur and spotting is rare where the sills have been intruded into grey slate ; but in the purple and green slates spots develop readily and they nowhere change to adinole. Adinoles. The porcellaneous rocks, which are developed from the grey slates, are rich in soda, and resemble the contact-altered slates which are described by German petrographers as adinoles. This term will be used here to describe them. Adinoles are seen wherever quarrying operations have cut back the sills, and in cliff and river-bank sections. In thickness they are usually from one to four feet, passing outward into unaltered slate, but at their junction with the diabase they resemble the igneous rocks so closely that it is very difficult to distinguish one from the other .without the aid of a micro- scope. The best exposure occurs at Dinas Head near Padstow; other occurrences are at Rock and Cassock Hill Quarries on the Camel Estuary, Lundy Beach in Portquin Bay, Treganna Quarry, and the Town Quarry, Wadebridge. East of Wade- bridge good examples are met with at Egloshayle and Kestle Quarry, and in front of Croan House. Adinoles and spilosites do not occur together. At Dinas Head and Pentire Head lime- stone and radiolarian chert are found in association with the adinoles. Thin bands of limestone also accompany the adinoles at Eock, Treganna, and Wadebridge Quarries. At Pentire, by Rumps Point, the limestone contains small green garnets. In hand specimens the adinoles are pale grey, yellow, or creamy in colour, but this is probably due to crustal weathering, as the rock always possesses grey-blue cores when broken open. It is not acted upon by hydrochloric acid, and cannot be scratched by a knife. " The rock is often veined with carbonates or quartz, and sometimes stained with limonite. Under the microscope the adinoles consist of ' Quart. Journ, Geal. Soc, vol lix., 1903, pp. 408-428^ ALTEEED SLATES. 4§ a cryptocrystalline or microgranular aggregate of albite fel- spar, which gives a lower refractive index than the Canada balsam in which it is embedded. Rhombs of pleochroic car- bonates^ are seen in bands' traversing many of the specimens, and siderate is not uncommon. Muscovite and chlorite have not been observed in them nor have actinolite or anthophyllite been met with. A spherulitic variety is found at Dinas Head and is thus described by Dr. TealP " a remarkable spherulitic rock. Spherules measuring |-inch in diameter. The central portions, of the spherules are generally composed of crypto-crystalline material. The outer portions of radiating blades or prisms of felspar, presumably albite. Ferric oxide, probably resulting from the decomposition of a ferriferous carbonate, is scattered through the slide in irregular patches, and concentrated in veins." Several chemical analyses of these rocks have been published.' They have a high percentage of soda, and when compared with unaltered slates show an increase of silica and soda with a decrease of alumina, potash, and water. It has been suggested by many writers that the increase of soda is due to additional material entering the rock in a vaporous state or in aqueous solutions at the time of intrusion of the igneous rock. On page 46 the mineralogical and chemical composition of these igneous rocks show an abundance of soda felspars, so that they might quite well have yielded up a sufficient quantity of the alkalies to produce adinoles from a slate composed essentially of muscovite (see also p. 73). * Spilosites or spotted slates. The spilosites are foimd in contact with the dykes and sills. Usually the spots are quite small, but in one or two localities they attain a considerable size — the largest being found in purple slates near Dinham House, where some are |-inch long by ^-inch broad. At Stepper Point they are as large as peas and of pecu- liar interest, owing to a progressive development in them of chiastolite. The spots are usually ovoid in shape, and are harder than the slate in which they lie. They weather out as knots. In colour they vary very slightly, and are for the most part dark grey or almost black. Under the microscope they are seen to differ from the groundmass of the rock in being darker, and in containing more qiiartzose and felspathic granules along with abundant dark pigmental matter and chlorite. These materials compose the spots almost to the entire exclusion of sericite, but are not uniformly dispersed over the spot area, as there is a peripheral ring which is darker than both the interior portions and the areas around the spots. Sheaves of chlorite occur in small spherulites. Besides this zoning there is develop- ment of dark rays along definite lines, which gives rise to various ' Eosenbnsch, Schroeder, Mikroskopische Physiographie, 1904, vol. 1, p. ii, p. 99. ' Geol. Mag., 1895, p. 18. ' Op. cit., p. 19. Trans. Roy. Geol. Soc. Cornwall, vol. xi, 1894, p. 723. 50 GEOLOGt OF tADSTOW. patterns resembling chevrons; and in some cases it forms the well-known crosses of chiastolite. The figures often have black squares at the centres and at the ends of the arms of the crosses, and this shows their relationship to chiastolite, but they are optically isotropic or nearly so. It is curious that these spots nowhere present the optical properties of the mineral, so that they have not reached the stage of mineralisation identifi- able as chiastolite. Spots of this character are confined to the variegated slates, and are especially well-developed in the purple varieties. Although the purple and red slates are so rich in iron ores, there seems to be very little iron or rutile in the spot areas. . Some of the slides show the serieite flakes winding round the spots, as in shear lenticles, but others seem to indicate that this effect may be due to the spots having pushed away the serieite from their margins during growth. 51 CHAPTER V. GEANITE AND LATER INTRUSIVE ROCKS. Granite. The granite moor rises higher than the adjacent country, but in much of its northern part this increased elevation is shown mainly hy the isolated hills and tors; there is here no sharp general rise at the junction with the slate. The western and southern margins, on the other hand, are always marked by a steep slope, often visible from a great distance. On these sides the junction of the granite and killas is a normal intrusive one; but towards the north end it appears often to be a fault. The boundary is of this nature two miles south of Camelford, and the position of the fault is marked by the great quartz-reef that crosses the foot of the valley of the Devil's Jump. The emer- gence, probably of the same fault, on the opposite side of the granite, is again marked by a quartz-reef, in this case so large that it is shown on the one-inch map (Lanlavery Rock). It seems probable that the straight northern margin of the granite near Parkwalls is also a fault. These faults appear to throw down the roof or covering of the granite, for they are accompanied by a sudden widening of the aureole of altered rocks. Faults also appear to throw down small patches of the roof within the granite area. The first of these, in the northern part of the granite, lies on the line of the quartz-reef already referred to, and its limits are indicated by fragments in the ploughed fields. The second occurs in the southern part of the area, a little north of the margin, on the west side of the St. Neot River. Small fragments of the altered killas were exposed there in the tip-heap of some recent trials, the actual surface of the ground being composed exclusively of blocks and debris of granite washed down the slope. The granite, as a whole, is divisible into three parts : (1) the most solid and free from cracks and joints, which forms the hills and tors; (2) the main mass of the granite, which is always more or leas decomposed at the surface ; and (3) the kaolinised granite described on p. 93. The more widespread portion of the granite, which has a reddish-brown colour, is often decomposed to a con- siderable depth. Wells have been sunk in it with a pick and shovel, without recourse to blasting, to a depth of about 40 feet. The rock, however, becomes coherent at a depth of 10 feet, or even less, and the wells do not often require walling below this depth. The rotted, brown, surface material, where acces- sible, is much used for binding the roads when newly metalled. The much fresher granite, forming the eminences, is a grey rock, large blocks of which weather out on the hills, where they are now found loose. Where specially solid, it forms bold crags, often almost vertical, knowp in the south-west of England as tors or earns. These tors are built up of lenticular blocks or sheets of granite, resting, as a rule, horizontally one above another. The sheets are at times of great size, as may be seen 62 GEOLOGY OF PADSt6\V'. at Eoughtor {see Plate III.)) Brown Willy, and to tte west of Bolventor, where, possibly, the largest blocks in the area occur. Quarries in the district further east have shown that their ellipsoidal form is not the rlesult of exposure; blocks of the same shape are met with for a small depth underground, separated by dirt and decomposed granite. This loose material is washed away when the rock is exposed by denudation, and the granite then dries and becomes harder, and is much better able to withstand decomposition. These masses of sounder rock, whether forming hills or tors, lie in groups, arranged parallel to the larger faults or master veins of the district, and are themselves singularly free from veins or faults.i' The Bodmin Moor granite, whether fresh or decomposed, shows little original variation either in composition or texture over large areas. A typical hand-specimen is composed of ortho- clase, in crystals about an inch long, quartz, in abundant grains of fair size, a somewhat subordinate amount of plagioclase, and white and brown mica. In addition, schorl may be said to be an essential constituent of the intrusion as a whole, though it seems to be absent at times from the marginal portions, notably from the rock of the De Lank Quarry. The distribution of the schorl in the normal granite will be best understood from the fact that it is impossible to find a specimen as large as one's hand that does not contain some schorl ; but a cubic inch may often contain none, and in consequence a' microscopic slide may show none. This schorl is clearly a product of original con- solidation. The orthoclase crystals, which give a porphyritic aspect to the granite, average about an inch long, by half an inch wide, and a third of an inch thick ; but there is, of course, a certain amount of variation in their size. Isolated, much larger crystals occur about Bray Down, in the north-east part of the intrusion; while in the central area there is a distinct tendency for the crystals to become smaller. The diminution in size is, however, far more marked toward the edges of the intrusion, though this feature is by no means persistent. In the few exposures that do occur of the actual edge, the rock is always finer grained, but microscopic sections show that the groundmass is still granitic in texture. A large portion of the north-west end of the granite tends to be persistently finer, but this may be explained by the fact that this part of the granite must often be close to the original top; about Pencarrow, where the phenomenon is specially well- marked, it is clear that we are dealing with the roof of the intrusion. Like all the Cornish granites, that of Bodmin Moor often shows a certain amount of parallelism in the arrangement of the felspars; where this is conspicuous, patches of the rock are much finer in texture, and the felspars . are _ rather rounded, as if there had been a certain amount of breaking up of the larger crystals when the "matrix was in a pasty condition. ' These harder masses are shown on the six-inch manuscript maps deposited inthe Library of the Geological Survey. The tors are illustrated by photos 476-485, 488 in the Survey Collection of English photographs. GEANITE. ^3 Quarries in the sounder granite of the hills and tors are rare ; indeed, the only good one occurs at Carbilly Tor, on the south side of the De Lank Water. There are two reasons for this scarcity of quarries; first, the difficulty of transport, and secondly, the freshness of the large loose hlocks on the hillsides. These are broken up and dressed on the ground hy the quarry- men, and only the finished material removed. Thus large fragments of sound rock are left, and afford good material for examination. Excellent specimens of the tor-forming granite may be obtained from a small crag, north-east of Millpool, in the Cardinham area. It lies just above the bend in the small stream and is both accessible and easily found. The rock shows well the typical white orthoclase crystals, about an inch long, and the coarse grains of free quartz. A section (7885) shows it is composed of orthoclase in large sheets as well as smaller patches, most of which show perthitic structure. Inclusions in this felspar are numerous, quartz being especially common. Quartz, often in large grains, is abundant and contains the usual fluid cavities with mobile bubbles and small cubical crystals. Both oligoclase and albite are present ; the former is more common and occurs in good-sized crystals, the inner part of which is generally decomposed They have, in most cases, an outer fringe of much fresher and slightly more acid felspar, of later growth than the interior. White mica occurs in fairly large, though rarely idiomorphic, crystals, as well as in numerous small flakes and spongy outgrowths of later development Brown mica seems less abundant ; it has the usual dark spots and the same colour as the brown micas in the other Cornish granites, and like them contains a certain amount of lithia. Sagenitic rutile, zircon and apatite are present. The slide shows also several grains of reddish pleochroic andalusite, but no tourmaline. The marginal portion of the granite, like the main mass, consists of more or less decomposed material with patches of much sounder rock scattered through it. A good general idea of the conformation of the ground formed by the two phases may be obtained from the high-road from Bodmin to Launceston, where it crosses the western margin of the intrusion. A small tor of sounder rock is seen on the south-east side of the road, and a steep slope marks the margin of the granite ; to the north-west the slope is much lower, and a well at Thome, close by, passed through a considerable thickness of quite soft and rather fine- grained granite before reaching any coherent or solid rock. Specimens showing this finer rock may be obtained from the marginal portion of the granite, close to the river, some three miles south of Camelford. A specimen (7889) from the wood north of Pencarrow is fine grained and non- porphyritic, with a gneissose foliation. This rock is rich in muscovite but contains very little biotite and no tourmaline. It has been much crushed and the micas have a parallel orientation, while the quartz is reduced to a mosaic of fine interlocking grains. Two large quarries have been opened in the marginal portion of the granite where it is more solid. Both show the faint foliation, parallel to the sloping margin or side of the intrusion, that is often present in the granites of the south-west of England ; but otherwise the rocks at the two quarries difEer considerably. The first, known as the St. Breward quarry, lies a little below the village of Row. Here the granite is much the same as the wide-spread rock ; it contains schorl and is traversed by several veins, coarse and fine. The second is the well-known De Lank quarry, in which the rock is remarkable for its homogeneous character, its freedom from veins or blemishes of any kind, and the cleanness with which the master joints out it into large blocks of exceptional soundness. Careful macroscopic examination of the broken frag- ments shows that schorl is so rare as to be practically absent. The grey granite of De Lank quarry contains porphyritio felspars up to an inch in length and has a parallel structure which is mainly the result of fluxion during intrusion. The large felspars are perthitic orthoclase, and often enclose oligoclase, biotite and muscovite. Black and white mica are present in nearly equal quantities and very frequently occur in parallel growth, the muscovite being external. The biotite has strongly marked pleochroic halos, 54 GEOLOGY OF PADSTOW. and the mnscovite often occurs in ragged flakes scattered through the felspaH and quartz. Oligoclase is rather common, having turbid weathered centres and a, clear border of albite ; the potash felspar is always perthitic. Tourmaline is seldom present and the accessory minerals are apatite, magnetite and zircon. Slight movements have attended the consolidation of the rock, as the quartz is often somewhat granuUtic and the white mica is torn out into wisps, but the felspars are seldom crushed. The Bodmin Moor granite has all the characters that distinguish the granites of Cornwall and Devon, but presents also a few features that give it a distinct individuality. It is comparatively poor in tourmaline, and in no case has it been proved to contain topaz, a mineral especially common in the adjacent St. Austell mass. Andalusite, on the other hand, is fairly wide-spread, hut cordierite is distinctly rare. Oligoclase is more common than in the other Cornish granites. Very distinctive also is the frequency with which cataclastic structures can be detected, especially in the marginal parts of the Bodmin Moor granite. This leads occasionally to the production of rocks which might be described as granite gneisses. ■ In order to obtain sufficient brown mica for analysis a large number of fragments of the De Lank granite were pounded up. It proved difficult to separate the biotite from the muscovite with which it is frequently undergrown, and, to eflect this, repeated precipitations in cadmium borotungstate were necessary. The biotite which is black in colour, appears dark-brown in thin section, practically uniaxial, with a pleochroism ranging from deep reddish- brown to clear yellow. The analysis (I) of the separated mineral was made by Mr. E. G. Radley in the Survey's laboratory. I. II. m. IV. SiOs l^iOs AUO3 FesOs PeO MnO CaO MgO K2O NazO LisO H2O (comb.) F 36-23 , 2-50 21-78 1-95 18-23 0-65 0-82 4-20 7-87 2-07 0-24 3-30 0-79 3992 22-88 2-32 15-02 1-40 0-68 1-07 9-76 0-99 1-71 SiPi 3-04 37-93 24-89 7-85 14-87 0-28 8-64 0-40 0-77 4-23 34-90 23-27 2-56 20-87 1-20 4-32 6-94 2-01 3-60 Less for F 100-63 0-33 98-79 99-49 99-67 iOO-30 This analysis proves that the biotite is not particularly rich in lithia or in fluorine. It contains much iron in the ferrous state and comparatively little magnesia. The second analysis (II) of a dark mica from Carn Bosavern, near St. Just, by Professor Haughton,' is closely comparable with that of the De Lank biotite, and serves to show that the biotites of the Cornish granites are fairly uniform in type. Analysis (III) shows the composition of a black mica from St. Dennis, Cornwall, analysed by Rammelsberg,* which is notably richer in fluorine. The De Lank biotite is very similar in chemical composition to the black mica of certain granites and granite-gneisses of the Scottish Highlands. The fourth analysis (IV). of the table was made by George Barrow' of a • J. H. Collins, 'Mineralogy of Cornwall and Devon,' ,1871, p. 62. ' C. P. Rammelsberg, ' Handbuch der Mineralchemie,' 1875, p. 582. ' G. Barrow, 'On an Intrusion of Muscovite-biotite-gneiss,' Owart. Journ, Geol. Soc, vol. xlix, 1893, p. 355, , ^ GEANITE. 55 mica separated by him from the intrusive muscovite-biotite-gneiss of Forfar- shire : it is almost identical with the analysis of the De Lank biotite'. Barrow points out that this mineral belongs to the Haughtonite group which was established by Heddle' to include those varieties of biotite which are rich in iron and poor in magnesia. Finer Granite. The Bodmin Moor granite is further linked to the other Cornish granites by the occurrence within it of several good-sized masses of finer granite, which are clearly intrusive. As a rule, the potash-felspars in the finer granite are quite small, but sporadi- cally they become larger, and at times are fairly numerous. The characteristic feature of the finer granite is the almost complete absence of the coarser grains of quartz, so abundant in the normal rock, whatever be the size of its potash-felspars. The first mass occurs near St. Breward, close to the western margin of the main intrusion. Its outcrop is rather obscure as the rocks here are much decomposed ; but its intrusive nature is well shown by the dyke-like prolongation from its south-western margin, which is cut open by the De Lank River at the north end of the famous granite quarry. The second occurs a little west of Bolventor, about the centre of the main granite. On the south and south-west edges of this intrusion loose blocks and at times the solid rock are a perfect melange of normal granite and finer veins. The third and much the largest mass occurs about and to the east of Dozmary Pool, extending into the area to the east of that here described. The veining of the adjacent normal granite is exceptionally well seen, in rocks in situ, at the south end of Browngelly Downs. This third intrusion possibly widens down- wards, for similar veins appear sporadically and are locally numerous for a distance of at least a mile west of the edge of the main outcrop. These veins often form short dykes, but they can be distinguished from the true elvans by their greater softness and the fact that they never have compact or felsitic-looking margins. An exceptionally large isolated dyke-like mass of this finer material crosses Roughtor and is shown on the map ; there are many lesser intrusions, but they are too small to be shown and often are represented only by a few scattered blocks. As is usually the case in these finer granites, schorl is per- sistently present and, on the whole, in greater quantity than in the normal granite ; the individual crystals, however, seem to be rather smaller. Sections of these rocks suggest that the material of which they are formed may have remained molten for a long time and at a great depth, for the earlier formed crystals appear to have been more corroded than similar crystals in the main intrusion, and, further, they appear to have had several epochs of growth. The structure, in consequence, resembles that of the aplites. Oligoclase is less common in these rocks, and muscovite appears in greater quantity than in the coarse granite; when ' M. F. Heddle, ' Chapters on the MLaeralogy of Scotland. Chapter fifth, — The Mio^s,' Trans. Roy. Soc. Edin., vol. xxix, 1880, p. 37, 56 GEOLOGY OF PADSTOW. tourmaline is frequent biotite may be absent. No andalusite was seen in any of the slides cut from tlie finer granites of Bodmin Moor. Occasionally tbe quartz is broken up into a fine niosaic b;v' crushing. Granite Veins within the Granite. Small veins of granitic material are common in many parts of the granite; they are not easily studied as they are most abundant in the moderately decomposed granite and by no means common in the solid granite of the tors, where alone they could be seen in situ. These veins vary somewhat in composition, but mostly contain a fair amount of schorl and little brown mica. Where coarsest they are essentially pegmatites, in which the crystals of white mica and schorl are of considerable size, but very irregularly distribtited. Several veins of this type are seen in the St. Breward granite quarry near the margin of the intrtision. From coarse rocks of this nature there is every gradation to a finer type, such as occurs in the blocks near the high road some- what west of Bolventor. Some good specimens of veins, in excel- lent preservation, occur in the quarry in Carbilly Tor, about two miles south-east of St. Breward. Granite Veins in the Rocks around the Granite. Small granite veins have been met with outside the margin of the main intrusion, wherever the ground is clear enough to enable this zone to be accurately examined. They tend to assume a low- hade sill-like form, though at times they ascend almost vertically. They are never numerous nor of great thickness ; the largest is probably less than two feet, while, as a rule, they are less than a foot thick. The ground is not clear enough to show definitely how far they extend from the granite margin, but the evidence suggests that they do not occur more than a hundred yards from the granite. Though always much finer in grain than the parent rock, they never have the flinty or quickly cooled edges of the true el vans. These veins are seen in the road-metal quarry, above Millpool, north of Cardinham ; in the bed of the stream above Blisland and again in the stream below Row, south of St. Breward. They are possibly most numerous at the foot of the bank on the west side of the Camel, opposite St. Breward. Modifications of the Granite. In various parts of the granite area and especially at the foot of the western slopes of Roughtor loose blocks of two varieties of granite may be seen, one of which contains much green chlorite, while the other has black radiating crystals of schorl in a red matrix of quartz and felspar, but is free from mica. These varieties have apparently undergone a distinct type of alteration which probably is connected with the process by which the tin- veins were filled, GRANITE. 57 Schorl-rock or Black-peach. — Fine schorl-rock or black-peach is met with over much of the granite area, but is very unevenly dis- tributed. It occurs as small veilis, varying in thickness from two feet to less than an inch. The indestructibility of the peach causes it to weather out in loose blocks, which occur in abundance at several localities, notably about the high-road, near the western edge of the granite. Here they are lying in great numbers on the uncultivated ground and many have been gathered up and built into the neighbouring walls. These veins continue to be abundant for some distance to the north and may be seen in situ in the old china-clay pits near Blisland. Further north they almost disappear and have proved very rare in the china-clay works of Stannon Marsh. On the whole the schorl-rock is rare in the northern part of the granite and more abundant in the southern and south-western part; it is always rare in the granite of the hills and tors. It is intensely hard and consists of a very fine admixture of quartz and schorl. Many specimens of this material have been pounded up, but no trace of tin has been found in them; indeed, their barren or ' hungry ' nature is well known to the miners. In the southern part of the granite, where the lodes occur, small fragments of softer peach may be found, and when of a grey tint may contain a little tin ore. The coarser, or more open-textured, types of schorl-rock so often met with in the St. Austell and other Cornish granites are distinctly rare in the Bodmin Moor mass. Greisen. — In the clear coast sections of the Scilly Isles, similar peach veins are often seen to be fringed by a thin band of granite altered to greisen, and doubtless the same occurs within the Bodmin Moor granite, but in the almost total absence of clear sections the point is difficult to prove. Occasionally, however, the greisen band is broad enough to form small blocks at the sur- face, and a fair number of these occur on the ground to the north of Dozmary Pool. Quartz-veins . — Quartz-veins are locally abundant in the granite, but they differ somewhat from the compact white reef-rock so common outside the granite. In most cases the whole of the vein is not white, but a considerable portion of it is tinted and the most common colour may be described as amethystine. Not un- commonly these veins closely approximate to an agate in structure and are composed of parallel bands or layers of finely crystalline quartz passing into chalcedony and more coarsely crystalline quartz, in the centre of which is often a narrow cavity,; along the edges of this cavity the quartz forms pyramidal, amethystine crystals. Specimens showing this agate structure are not un- common about the area to the south of Bolventor ; and the amethy- stine quartz is common in all the refuse heaps of the old china-clay works to the south of the high-road. Quartz-veins, showing a tendency to agate structure, occur sporadically over the whole of the granite, except the southern portion, where the workable lodes are situated. The evidence as a whole suggests that the conditions favourable for one type of vein were unfavourable to the develop- nient of the other. Oo GEOLOGY OF PADSTOW. " ' ■ ElVANS. -■ Numerou.s elvans, or quartz-porphyry dykes, are associated with the Bodmin Moor granite, but they are unevenly distributed. One only has been met with alDOUt the northern end of the mass, and for some distance to the soutli of this there appear to be none. They are more frequent about the i)e Lank Water and the Blisland area, but are most abundant toward the southern end of the great intrusion. Two or three extend westward towards Padstow and Wadebridge. The elvans are sliglitly newer than the granite which they cut ; in no case does an elvan form the continuation or prolonga- tion of the gfanite margin. The junction of the two rocks has been laid open in numerous quarries, and these show that the edges of the elvans are always composed of fine-grained, * fel- sitic ' materiiil, even when in the heart of the granite mass. The actual junction of the elvan and granite is always sharp. The centre of the elvan is always coarser in texture than the edge, and when the rock is porphyritic the phenocrysts in most cases steadily increase in size with increasing distance from the margin. There is considerable difference in the composition and structure of tbe elvans ; some are markedly porphyritic in the centre, crystals of orthoclase and quartz being easily recognised. In some cases the felspar alone attains considerable size, but it is rare to find good-sized crystals of quartz unaccompanied by felspar. Macroscopic brown mica is common in some elvans, but rare or absent in others. White mica, as an original constituent, can rarely be dis- tinguished by the unaided eye, though often present in the rocks. Macroscopic pinite is at times abundant in the elvans of the Blisland area. The matrix or groundmass varies considerably; sometimes it consists of a microgranitic aggregate of orthoclase and quartz; more commonly it is micrographic, often with fine granophyric spherulites, and in this case plumose wbite mica is abundant. In the southern part of the area it is often impossible to determine the original characters of the elvans, for they have been greatly altered, and are now composed largely of white- mica. They have undergone the same change as the granite when it is altered to greisen. These rocks are friable and quite useless for road metal. They are often porous, and this cbaracter seems in part, at least, due to the weathering out of pyrites. In the few harder specimens that have been found the highly micaceous elvans contain a great number of cubes of pyrites. The alteration appears to be connected with the infilling of the tin-veins, for in at least two instances these elvans have been mined for tin. The solitary dyke met with at the north end of the granite is unusually persistent. It has been quarried at a considerable number of points, and in all cases is very pale in colour, almost white; but only two good quarries are worth examining now. The first is at the foot of the bank on the east side of the Camel, about one mile south of Camelford, and here the soundest rock occurs ; the other is oji top of the bank on the w^st of the river, ELVANS. 59 The latter shows all the typical features of an el van, both as regards field relations and composition. It may be conveniently called a quartz-porphyry, for in the centre of the dyke the quartz-crystals are easily seen. An additional and special feature is the occur- rence of films of fluorspar in cracks near the margin. The rock contains porphyritic muscovite in addition to quartz, orthoclase, and oligoclase. The microgranitic groundmass consists of primary muscovite, quartz, and alkali felspar. Much further south, near the centre of the west edge of the granite, an elvan occurs at the south end of the De Lank quarry. As there are exceptional facilities for transport it is much tPorked,' and fresh specimens can be obtained. Its groundmass contains much spherulitic micropegmatite along with scales of primary muscovite. On the south side of the De Lank "Water, nearer its junction with the Camel, there is a group of elvans which may possibly be connected at no great depth, for they all have a somewhat sill-like mode of occurrence. A quarry has been opened in one of these close to the most southerly point of the broad mass of alluvium shown on the map ; it is probably the freshest elvan in this country. It is a good example of the grey type of elvan, with macroscopic biotite and a few phenocrysts of quartz and felspar. White mica occurs almost exclusively as minute fans which form part of the groundmass, and not as phenocrysts. The matrix shows beautiful granophyric structure, exceedingly fine-grained and spherulitic. The elvans about Blisland contain numerous macroscopic Crystals of quartz and felspar, and the matrix contains much of. the fine. muscovite to which Dr. Flett has often drawn attention.* " Some distance to the south-west, in Shell Wood, a quarry was opened in a small sill-like elvan, considerably decomposed, but containing, apparently, a fair amount of pinite. The latter is specially abundant in the small elvan trending north-west, near Txewardale. The outcrop is largely represented by loose blocks, but in these the six-sided prisms of pinite are often common and easily seen in the hand-specimens. An exceptionally interesting elvan occurs on the west side of the high road, across the granite, and a little west of Temple (5527). A quarry in this shows numeroiis patches of fluor-spar, varying from violet to amethystine in tint. Far more important, however, is the occurrence of minute films of kaolinised material along a few crush planes. These show that the kaolinisation seen in the adjacent china-clay pits afEected the elvan to some extent, and was, consequently, later than the elvan. The films of china-clay coat planes suggestive of low-hade fractures pro- duced by great force, and it seems probable that the gases that caused the kaolinisation may have been forced up under far greater pressure than is commonly supposed. An elvan has been tra,ced for a considerable distance across G^rdinham Moor, where it cuts the granite, almost to Millpool, where it is in the killas. Close to its eastern termination there • ' Geology of Bodmin and St. Austell ' {Mem. Geol. Surv.), 1909, p. 78. 15999 E 60 GEOLOGY OF PADSTOW. is a quarry for road metal, which shows perfectly the typical mode of occurrence of an elvan, and the rock itself has a normal composition and structure. . The great bifurcating sill-like elyan, ahout and west of War- leggon, is also of the normal type. Another singularly fresh example of the elvan occurs close to the greenstone, to the north of St. Neot. The extreme freshness of this rock is remarkable, as it occurs in the midst of a group of elvans that are just as remarkable for their extreme alteration. An elvan of peculiar habit and petrographical characters traverses the country from near Camelford to the sea south of Bray Hill, opposite Padstow — a distance of 12 miles. Near the village of Trewethern it divides into two branches, which pursue independent courses for half-a-mile and then reunite. Near Bokelly the elvan forms a bold ridge of hills, which rise abruptly from the gently undulating country near St. Kew; and by this feature alone it can be traced for a distance of two miles. Near Pityme it is buried beneath the blown sands, but is quarried in pits dug through the sands. It is last seen in the clifE-face a little south of Bray Hill. The stone supplied material for building the railway bridges between Wadebridge and Camelford, and is much employed for building chapels and halls. In the hand-specimen the rock is light brown, often with green oval blotches, is fairly soft, and can be trimmed with an axe. Under the microscope it is seen to be allied to the minettes, but is usually decomposed and permeated by calcite ; all the biotite is green with decomposition products arranged along the cleavage planes. Near St. Kew globular inclusions of aplite, over a foot in diameter, are seen in the large quarries. At this locality the joint planes dividing the dyke are curved. Another interesting elvan is quarried for road metal in Bod- rigan Wood, a mile west of Blisland. This rock possesses granophyric structure in its groundmass, and has claret-coloured, undulose biotite. South of this locality, in Tresarrett Wood, an elvan of normal character forms picturesque crags by the river side. A porphyritic elvan, which contains topaz and pinite with large piiak orthoclases, is found near Fenterlarick. Altered Elvans. The first of the elvans that attract notice as being abnormal occurs in the bed of the stream that flows into the Camel to the north-west of Helland. Its outcrop is not seen, the elvan being represented by a line of large loose blocks. The main group of altered elvans commences with one on the north side of Cardinham, but the alteration is better marked in these rocks at the north-east of Treslea Downs. Further east it is exceptional to find an unaltered dyke. Two types of alteration can be recognised in the elvans of this area, as in other districts of Cornwall. The first is essentially the same as that which affects the granite when it is converted into greisen. In this process the felspar of the original rock is EL VANS . 61 replaeed by aggregates of white mica and quartz. It should be remembered that many of the Bodmin Moor elvans contain primary muscovite, both as phenocrysts and in the groundmass, but in these rocks the felspar is in good preservation. The g;reisening first attacks the groundmass, and the porphyritic felspar often remains after that of the matrix has completely disappeared. At first the new mica is in exceedingly minute scales, embedded in secondary quartz, and the shape of the felspar grains is retained by these aggregates. Subsequently, the size of the mica flakes increases, and the groundmass is replaced by a granular mass of quartz mingled with scales of muscovite. Often the new quartz is deposited in optical con- tinuity on the original quartz phenocrysts, forming secondary halos. In the granophyres the original spherulitic structure is sometimes indicated by radiating lines of fine dusty inclusions in the secondary quartz. In the Park Wood elvan a little cassi- terite has been deposited in minute brown grains (7901). The second type oi alteration is tourmalinisation, and resembles that by which the granite passes into luxullianite and granular schorl rock. The Hobb's Hill elvan has been tourmalinised ; in its matrix the felspar has been replaced by radiate nests of blue schorl prisms embedded in quartz (7908 — 9). Some speci- mens still retain the porphyritic felspars; in other these, too, have vanished. The original brown tourmaline is surrounded by secondary blue tourmaline. A good deal of white, mica appears in the ground-mass, and in the patches of blue schorl large numbers of small brown grains of tin stone may often be seen. The porphyritic muscovite remains, but the orginal biotite has disappeared, and its components have been used in the formation of the secondary tourmaline. MiNETTES. Three dykes of mica trap are seen 400 yards north of Lemail Farm and in the railway cutting a mile west of St. Mabyn. The thickest band is between 25 and 30 feet. It can be traced in the cutting breaking across the cleavage of the slates and appears on the eastern side in undulating bands which weather to spheroidal masses. The central part of the dyke contains large porphyritic biotites set in a finer-grained matrix ; the margins consist entirely of the fine-grained rock. Where the slate is in contact with the minette it becomes baked and hardened for a distance of about three feet. Hand specimens are purplish-grey in colour spangled with numerous plates of biotite. Under the microscope the most con- spicuous mineral is the deep brown mica with darker exterior zones. Serpentine pseudomorphs; -apparently after olivine, are the other ferromagnesian constituents. The ground-mass consists of smaller biotite plates and turbid ortho^lase. There is also a large quantity of calcite in the rock, together with some quartz and apatite. Two other examples of minette occur on the banks of the river Camel near Padstow; one on the south bank at Halwyn and the other on the north side, south of Gentle Jane 15999 ^ 2 62 GEOLOGT OF PADSTOW. n6ar Cant Hill. Both these rocks are vesicular, decomposed and cOlisist of calcite with numerous pseudomorphs after biotite scattered through the mass. The following analysis (I.), made by Mr. E. G. Eadley in the Survey laboratory, may be compared with that of the Gaiinei minette, from near Newquay, with which it presents very close similarity of composition : — SiO, TiO, ... AUOs ... FejOa ... FeO MnO ... (CoNi) O BaO SrO CaO MgO ... K,0 Na,0 ... Li,0 H,0 at 105° C. H,0 above 105° C P,Os ... FeS, ... CO, F CI Total ... Less O for P and CI Total ... I. Minette, Railway cutting near Lemail Farm, one mile west of St. Mabyn Church, Cornwall (anal. E. G. Radley). E. 6687. II. Minette, Gannel Quarry, Pentire, Newquay (anal. W. Pollard). 'The Geology of the country near Newquay' (Mem. Geol. Surv.), 1906^ p. 61. Though these dykes are nowhere seen to cut the elvans, they are probably of more recent date, and may be connected with the trap-rocks of the Exeter district, which occur near the base of the New Red Series. I. n. 49-14 50-98 1-81 1-25 14-89 16-13 108 4-20 3-88 3-24 •14 •17 -08 trace -49 -20 nt. fd. trace 5-13 5-50 707 , 7-28 5-82 4-82 2-74 2-99 trace trace -15 •44 116 1-46 1-49 -74 -32 -43 4-94 •58 ■20 -05 •07 100-58 -10 100-48 100-48 ■02 100-46 63 CHAPTEE VI. AUEEOLE OF THEEMOMETAMORPHISM SUEEOUNDING THE GEANITE.' The Bodmin Moor granite is surrounded by an aureole of thermometamorpliosed rocks like those of the other Cornish granites. As usual there is a certain amount of variation in the breadth of the aureole, depending mainly on the underground slope of the granite surface. Where this plunges down steeply the breadth of the aureole is necessarily less than when the inclina- tion is gentle, and the granite extends under the sedimentary rocks at no great depth. Along the southern and much of the western margin of the intrusion the alteration of the killas can be traced to a distance of about three-quarters of a mile from the granite; to the north of the fault at the Devil's Jump the aureole rapidly increases in breadth as Camelf ord is approached, being fully a mile and a half wide at Lanteglos. This increased breadth is maintained all along the northern edge, but on the east side of the mass near Bray Down the aureole resumes its normal dimensions. The most highly altered and crystalline rocks are confined to the immediate margin of the intrusion, and the component crystals of the altered rocks rapidly diminish in size at a distance of a few yards from the margin, no matter whether this distance be measured horizontally or vertically. It may be that rocks at some distance (say 500 feet) above the granite are more altered than rocks at the same distance horizontally, but no eyidence has been met with to justify this view. During the progress of the survey it was found that the rocks on the higher ground are usually much decomposed, and their original composition can be ascertained only by a comparison with specimens obtained from the bottom of valleys that have been recently deepened ; where this deepening is considerable there is little difficulty in finding moderately fresh specimens. Where the altered rocks occur on the steep slope close beside the edge of the granite they are much less decomposed, but in this case they are unfortunately often buried under downwash. Eecent deepening has taken place in the following valleys : — (1) St; Neot, in the south-eastern part of the area ; (2) the valleys on each side of Warleggon, more particularly the western one; (3) Millpool, where rocks close to the margin may be obtained; (4) Blisland ; (5) De Lank, above Keybridge ; (6) the small valley below Eow, where the actual junction with the granite is exposed ; (7) the small valley opposite Gam Bridge; (8) the Camel from Wenfordbridge to Slaughter bridge. Good exposures are extremely numerous in the Camel Valley, not only near its base but on its flanks. The river passes twice completely through the aureole and the actual contact with the granite is well shown on the east bank opposite Michaelstow Church. In other places good material is difficult to obtain except from quarries, which are few in number. > By Dr. J. 8. Flett and G. Barrow. i64 GEOLOGY OF PADSTOW. The altekeb K-illas. The bulk of the killas within the aureole was originally a very fine micaceous mud, its variation in composition depending mainly on the increase and decrease of the amount of white mica and ohloritic material present. Where the former is greatly in excess the altered rock is silvery-grey or almost white ; as the proportion of chloritic material increases, the altered rocks become darker and the change is accentuated by the fact that they also contain much more finely- divided iroii-ore than those that are rich in white mica. In the original fine killas the outlines of the clastic grains of quartz and felspar are often not well defined, while clastic micas are either very small or are absent, having undergonie complete recrystallization. The unaltered killas was composed largely of minerals which contain much water of hydration. This water of hydration is considerably in excess of that present in the perfectly fresh micas of an igneous or metamorphic rock and can be driven off at a much lower temperature. The first effect of. the granite intrusion was to set free this water in the form of superheated steam, which digested the original components of the rock; from this digested material new crystals subsequently separated out, that now compose the metamorphic rocks within the aureole. The metamorphosed killas has almost always a foliated struc- ture, and this is specially marked near the south and west edges of the granite where the rocks have suffered to an exceptional extent from dynamic action. They have not only been folded and cleaved, but also intensely sheared and cut up into lenticles of varying size. The process is accompanied by the forcible separa- tion (segregation) of much of the finely divided silica of the original rock, which now occurs in lenticular patches, varying in thickness from a mere film to about half an inch. Much thicker segregations have been met with, but they are uncommon. These folding and deforming movem.ents were all anterior to the intrusion of the granite, as may be seen by examining its junction with the killas. This certainly cuts off all structures in the killas that are due to dynamic action ; the granite is affected only by comparatively straight faults that also traverse the killas next it. The quartz segregations become more crystalline^ as the granite is approached, and they must also have been produced before the intrusion took place. The thermal action of the granite does not obliterate the struc- tures due to dynamic agencies, but in the groundmass tends often to accentuate them. Thus' the parallelism of the micaceous material produced dynamically in the unaltered killas is repro- duced in the groundmass of the altered rock, where the white micas are seen, under crossed-nicols, to form a brilliantly ' de-" polarising ribbon,' composed of minute flakes with a parallel arrangement. The killas seems to have been* left in a state of intense strain parallel to the foliation, and this strain dominated the building up of the new crystals of the groundmass. The- most -abundant rocks within the aureole - consist of metamorphosed shales. With them are associated calcareous ALTEEED KILLAS. 65 rocks of variable composition and at least one sill of greenstone. Several types of altered killas may be recognised, of which two are of principal importance : — I. Those in which the white mica greatly exceeds the brown, so that the fresh rocks have almost a silvery aspect. In the hand specimens these rocks usually show distinct spotting when they are within the aureole and the spots are darker than the matrix, often greenish and usually elongated parallel to the foliation. They vary in size up to a quarter of an inch in length. In the vicinity of the granite these rocks may contain andalusite, but cordierite has not been observed in any of them. The spots con- sist of dense aggregates of fine-grained white mica mixed with green chlorite and are less coarsely crystalline than the matrix. It is possible that some of these spots represent weathered anda- lusite, but that mineral has not been found in the interior of any of them, and in some rocks andalusite occurs in a very fresh con- dition alongside of these spots and independent of them. Many of these silvery schists contain small crystals of green chloritoid of fairly large size showing rectangular outlines in vertical section with polysynthetic twinning. They often lie across the foliation of the rock and are visible in the hand specimen as dark green specks. Rocks of this type may be called chloritoid mica schists or chloritoid phyllites. They contain also small plates of iron oxide and occasional prisms of rutile. In some of the specimens the micaceous matrix has a perfect parallel arrange- ment with simultaneous extinction over large areas. To this structure has been given the name ' depolarising ribbon.'^ The micas in the spots have the same arrangement as in the matrix and the dark colour which the spots exhibit in the hand specimens is due to the greater abundance of chlorite in them. A large area of rocks of this type occurs on the north of the out- crop of the calcareous beds near Camelford. They are the altered representatives of the pale greenish-grey and buff Upper Devonian slate of that district and lie above the calc-flintas. Good speci- mens may be obtained about Trenuth and a steady decrease in metamorphism may be observed with increasing distance from the granite boundary. The spotting is also visible at Trefrew, but disappears before Slaughterbridge is reached. A smaller area of these rocks occurs to the south and east of Camelford, around Outground Mill; it lies between two outcrops of calc-flinta. Spotted grey phyllites are visible also in a quarry half a mile north-east of Lanteglos, and about Crowdy Marsh, on the east of Camelford. Large crystals of andalusite are present in a shining grey chloritoid schist from a well near Garmoe, Lane End (6478) ; the matrix of this rock consists very largely of white mica with a perfect parallel foliation. These rocks have already been referred to on p. 32. II. The second type of altered killas contains muscovite and biotite in approximately equal quantities. These rocks are of Middle Devonian age and occur on the south side of the long out- crop of greenstone shown on the map. Outside the aureole they are ■ BuriTow, ill ' Geology of Bodmin and St. Austell,' (Ifem. Geol. Surv.), 1509, p, 89., ' 66 GEOLOGrY lOF ifADS'EOW. fine phyllites passing into micaceous slates consisting ma,inly oi white mica, chlorite, quartz, and iron-oxides, without biotite, and in hand specimens they have a leaden-grey colour. In many speci- mens the cleavage due to parallel orientation of the mica and chlorite scales ia very close and perfect, but frequently it is crossed by a slip-cleavage at high angles, and the splitting surfaces are then wavy and puckered.^ Near the granite, however, they become more coarsely crystalline and at the same time assume a browner tinge owing to the development of biotite in place of chlorite. Very frequently they are spotted and the spots, which may be half an inch long, are usually darker than the surrounding matrix. Some of these rocks in which the micas are of considerable size might be described as spotted mica-schists, as the foliation persists or becomes emphasised through the recrystallisation. The more extreme stages of alteration, in which the argillaceous rocks are converted into a compact splintery hornfels, such as occurs fre- quently among the black inclusions of altered slate within the granite are not well represented in the aureole. The biotite of these altered rocks is of deep brown colour and intense pleochroism. By weathering it passes into chlorite and rutile. The white mica also forms irregular plates and these two minerals are usually grouped together in micaceous films separated by bands or lenticles of quartz. Although their arrangement is in general parallel to the foliation, larger crystals often lie across this with any orientation. Next to the micas the commonest mineral is quartz, in small grains fitting closely to one another; iron-oxides and rutile are also common. Felspar is rare in these rocks ; occasionally large platy crystals of chloritoid may be sieen in the microscopic sections. Andalusite is, on the whole, abundant in the more altered speci- mens. It forms prisms with quadrangular cross-sections and sometimes more than quarter of an inch long. Inclusions are not very numerous and are principally biotite, iron-oxides, and quartz. The brown mica is to some extent expelled from the andalusite during growth and gathers on its surfaces, but in some cases scales of mica lie in the andalusite and are parallel to those of the ground mass. Weathering produces aggregates of secondary mica from the andalusite and at the same time the mineral becomes brown or reddish. Cordierite is still more frequent than andalusite, and in fact is characteristic of the rocks of this group when the contact altera- tion is of a high order. It seldom forms large crystals with the well-known sector structure due to interpenetrating twins, though some instances of this occur (7814). For the most part the cor- dierite is in small grains entangled in mica or in quartz and not easily recognised when they are perfectly fresh, but, as this mineral decomposes more rapidly even than biotite, it is often represented only by pinite when all the other components of the rocks are in good preservation. The pinite is green or yellow and sometimes quite isotropic, but in many cases these aggregates con- tain doubly refracting material which seems to be secondary white mica and chlorite. ^ ^_ __ ' A description of the unaltered rocks will be found in Chapter II. , ALTERED KILLAS. 67 . A small amount of tourmaline is by no means uncommon, and though this may be original in some cases, it usually indicates ±hp operation of volatile substances emitted by the granite. Where tourmaline is at all abundant, biotite is scarce, the one mineral obviously replacing the other. In these rocks the spots are of several different kinds; many of them are not crystals but aggregates of scaly white mica, chlorite and iron-oxides like those above described. Other spots are crystals of andalusite, with dark incrustations of biotite. In a few cases the spots are larger grains of cordierite. Occasionally biotite, develops in fairly large crystals, but these are not of sufiB.- cient size to be conspicuous in the hand specimens. In no case was garnet observed in the rocks of this group. Blibland Area. Good opportunities of obtaining specimens showing the pro- gressive alteration of the killas as the granite is approached are afforded by the exposures in the bed and banks of the stream flowing past Blisland. Outside the aureole the rocks are well- cleaved slates with thin folia of quartz, white mica, and chlorite, showing with polarised light a very characteristic " ribbon ' Structure (7350). Slip-cleavage or cross-cleavage is frequent also and produces a rippled appearance on the splitting surfaces ; in some rocks with well-marked quartzose folia a lenticular structure is also visible (7356). The first effect of contact alteration is the development of brown mica in minute scales which lie parallel to the foliation; in weathered specimens the biotite is represented by chlorite. A faint spatting can be discerned in the killas almost a mile from the edge of the granite, but is not universal in the contact altered slates. The cleavage at the same time becomes less perfect, and, for that reason, the rocks are not quarried. Above the bridge near Blisland the rocks are distinctly more crystalline and near Cassacarn Mill they contain andalusite and cordierite. The andalusite occurs in long prisms with rectangular transverse sections and few inclusions ; the cordierite mostly in small grains but also in large irregular masses which may enclose andalusite. In these rocks there is much biotite and both micas have crystallised in fairly large plates, most of which lie parallel to the foliation, but many of them cross it obliquely or at right angles. Spotted schists occur in the lane below Tumrose farm, and at Trehudreth Mill, within a few yards of the granite, there is a well-foliated biotite-schist rich in fresh recrystallised alkali felspar. In the Millpool district to the north of Cardinham there are many good exposures. The first signs of contact metamorphism are visible in the highly-sheared slates in the lane that leads from the Bodmin road to Millpool ; about Freeze the rocks show a fine even cleavage. Andalusite is found at a distance of about 700 yards from the granite margin, and the rocks are rich -in brown mica. In a small quarry near Millpool specimens of a white silky micaceous phyllite may be obtained. This rock (7603) con- tains no biotite but much tourmaline in small deep brown prisms, 68 GEOLOGY OF PADSTOW. and belongs to the type which has been described as cornubianite. The tourmaline is evidently of pneumatolitic origin as the rock comes from the edge of a small vein in which trial shafts have been sunk. Nearer the granite the rocks contain both andalusite and cordierite (7801), as for instance, further east, at the bend of the stream, close to the granite margin and opposite Bury Castle (7346). In the area around Warleggon the rocks are rarely fresh. Spotted killas is common in the western stream above Panters- bridge, and at the north-east corner of Treslea Downs the rocks are much spotted and contain andalusite. Near Cabilla Farm- house specimens were collected in which both andalusite and cor- dierite are present. At Tremaddock, north of St. Neot, the spotted schists contain weathered cordierite and long prisms of andalusite which are sur- rounded by films of biotite and appear as black needles in the hand specimens. The railway-cutting connecting the De Lank quarry with the top of the incline above Wenfordbridge shows altered killas with abundant quartz segregations. The schistose character of these rocks tends to be emphasised by decomposition. The biotite in many of the specimens has weathered to chlorite. Andalusite is common and in some cases is decomposing to soft aggregates of fine mica which give bright polarisation colours. In many speci- mens there is also a yellow or green decomposition product after cordierite, and in a few of the specimens fresh cordierite is still present. Many of the large plates of biotite are set across the plane of foliation. Tourmaline, though often present, is not really common in these specimens. Spotted killas may also be found in the bank at; the north edge of the broad mass of alluvium shown on the map. Camblpord District. The River Camel, to the north of Wenfordbridge, twice passes completely through the aureole of thermometamorphism produced by the granite. On the west bank exposures are numerous as far north as Slaughterbridge, nearly two miles beyond Camelford, and as the river has recently deepened the valley through which it flows, good specimens often can be obtained near the margin of the alluvium. Except in the short stretch immediately below Trecarne, exposures in the river bed are not numerous; even when visible the rocks are more decom- posed than in the adjacent banks. On the east bank outcrops are not so clear, being much obscured by wash from the granite ; the rocks are, however, closer to the granite, and in two places the actual junction of the killas and the granite can be seen. On the west side the unaltered clean-cleaving dark slate is exposed in the banks and road-side just above Wenfordbridge. In the wood to the north a trench has been cut for the pipes from the Stannon Marsh Clay-works and large fragments of well- preserved rock can be obtained from the material thrown out. These illustrate well the extremely gradual change from the normal killas to the rock that has been metamorphosed. ALTERED KILLAS. 69 In the immediate neighbourhood of Penrose Burden the rocks are much the same as those about Cassacarn Mill, near Blisland, and range from the lighter-coloured fine leaden-schist with anda- lusite, through the darker-coloured material with much red-brown mica and large patches (spots) of andalusite, to an almost flinty- brown rock, rich in brown mica, but with little or no andalusite. Cordierite, mostly in a weathered state, is present in nearly all the specimens. High up the banks and in the adjacent field all these rocks show the lenticular aspect of a very fine schist ; at the foot of the banks, where they are much fresher, they have often a hornfels aspect on a cross-fractured face. To the north of Penrose Burden large spots (andalusite) are often present, but the rocks here are generally rich in red-brown mica and begin to be associated with the types allied to the flinty biotite hornfels that contains no andalusite and is associated with the calc-flintas. East Side of the Camel. On the east banks of the Camel the sections as a whole are not good, but a few are specially interesting, as they are close to the granite margin, and in two localities the actual junction of the killas and granite is clearly exposed. The first is in the small stream flowing down from the village of Row, to the south-west of St. Breward. Here the edge of the granite forms a waterfall and the specimen of highly altered killas, decribed on page 74, was chiselled off the granite. Further down, specimens illus- trating the decreasing metamorphism may be found, and, in addition, there are two snlall outcrops of granite veins. The second junction is met with where the granite margin descends to the alluvium of the river, as shown on the map. The contact rocks are clinging to the steeply sloping roof or face of the granite. At this point specimens in abundance may be obtained within a short distance from the actual contact. Calc-fiinta. Altered impure calcareous rocks occur within the metamorphic aureole throughout almost its entire length. Owing to their great tendency to decompose and disappear at the surface it is often impossible to trace their outcrop or form a true estimate of their extent. As the survey of the country proceeded it became clear that the calcareous bands were more or less persistently associated with two rocks, one of which is an altered greenstone sill, the other a shale of the type that, when metamorphosed, forms a flinty-biotite-hornfels. The calcareous material occurs in bands varying in thickness from a fraction of an inch to several feet; thin bands of an inch or two are by far the most common. The associated greenstone also occurs in bands varying from a few inches to many feet thick. It is so often accompanied by an adinole, due to its contact action, that the sill must have been fairly_ thick originally, and the.mere films of it can have .been produced only by a combination of folding and disruptive move- ments (overthrusting). The outcrops along the southern and 70 GEOLOGY OF PADSTOW. - '^ • we,steiii side are clearly disconnected patches, and, in conse- quence, it is impossible to say to what extent the map should show the presence of calcareous rocks in this part of the area. They are often not visible where they might be expected to occur; and their absence may be due either to dissolution or to cutting out. and suppression by overthrusting. The largest visible outcrop of the calc-flintas occurs in the Camelf ord area and is shown on the m^p. Near its northern margin a specially thick mass of this material has been traced from the quarries at the ford, about a mile east of Camelford (known as Tylaiid's Quarry), to the quarry at the head of the lane at •the north end of the town. This mass is believed to be the top of the group, as it is associated with some exceptionally dark flinty hornfels Jiot seen elsewhere. . , Further south the" types are mainly thin-banded. Exposures are numerous and specially good specimens can be obtained from (1) the roadside just below Pencarrow, about a mile due south of Camelford, (2) the quarry by the road below Trecarne, two miles south of Camelford, and (3) the bed of the river below Trecarne. About Michaelstow fragments of the altered calcareous rocks are met with in the ploughed fields close to the margin of some of the larger masses of green- stone, lu Lamphill Wood, south of Gam Bridge, these rocks form small scars ; here they are nearer the granite and well preserved. For some distance to the south little is seen of the calc-flintas, and in parts of the area the ground is sufficiently clear to show ihat they can be but feebly represented, the next outcrop, on the margin of the greenstone near Lank, being of quite small thickness. Near Blisland many small outcrops occur in the road to the north of the higher of the two bridges and in the lane leading to Polpry. On the southern margin of the granite a great number of small isolated outcrops occur over a somewhat oval-shaped area stretching almost from the bay-like indentation in the margin of the granite to the west side of Goonzion Down, near thei southern margin of the map. The rock furthest from the granite is mostly a peculiarly compact pale-grey rock with a fine texture and sub-conchoidal fracture. This type is best exposed in a quarry to the left of the road on Goonziop Down at the edge of the map ; smaller exposures occur on the east bank of the stream at Fantersbridge and on both sides of the road a little further east. Abundant blocks are seen loose on the boggy ground between the three farms of Pawton and on the adjacent farms. These are more crystalline. Considerably nearer the granite is an outcrop in the lane, a few yards south-east of Cabilla ; another is visible in the stream south-east of Warleggon ; while a much more massive rock was laid bare recently, close to the adit to Tregeagle Mine, north of St. Neot. Blocks of highly altered calc-flinta have been dug up near the margin of the granite to the north-west of St. Neot. These are all highly crystalline. The calc-flintas here described are at a different horizon in the Devonian to those previously described as occurring around the St. Austell granite^ and differ considerably in composition. In the latter garnet and idocrase were either rare or absent, but within the Bodmin granite aureole these minerals are abundant. A still more marked difference is the rarity in the rocks now described of minerals due to pneumatolytic aciion; these are extremely common about the St. Austell granite. The finest exposure of these rocks occurs in the two quarries on the sides of the road at the Ford, about a mile east of Camelford, which show not only the calc-flintas but also their associations with other rocks. In the more northerly of the two quarries the topmost five feet is composed of a fissile metamorphosed greenstone Below this is a more fissile band, three feet thick, containing much brown mica and minute dark spots. In this bed are three small bands of calc- flinta which thicken and thin rapidly ; they generally have a yellowish tint, but •> '' The Geology of the Country round Bodmin and St. Austell ' {Mem. Geol. Swv.), 1909. CAtC-FlINTA. 71 locally become pink owing to the great increase in the amount of garnet in them.- Lenticles of the flinty green (malacolite) rock also occur ; like the last they increase at the expense of the brown micaceous rock in which they occur. Below this again is a dark grey rock, six inches thick,' hard and massive when fresh, which readily decomposes to a soft reddish or brown clay ; it is a kind of altered mudstone. Below this again is some six feet of the hard greenish-grey flinta containing lenticles of yellow-pink material. Beneath this is about four feet of rock composed of alternating laminae of fissile altered greenstone and calc-flinta of very varying tint. This is underlain by six feet of more solid greenstone, though this can be split parallel to the banding of the rocks. At the base of the quarry is another interlamination of greenstone and cak-flinta. As there is no reason to doubt that the greenstone bands were all originally part of the same sill, one sees at once that these rocks have been literally cut up by earth movements, and this afEords the real explanation of the curious manner in which the calcareous rocks appear in force at one point and yet seem to disappear in a closely adjacent locality. A feature of the calc-flinta bands is the occurrence of premetamorphic cracks, or joints, into which a large amount of lime filtered. The local excess of lime has enabled lime silicates to develop in a matrix of calcite, which seems to offer little resistance to the formation of other minerals which are often idiomorphio in consequence. It was from these joints that the specimens were obtained that were described by Mr. Thomas in the Mineralogical Magazine. (Vol. xv, p. 113, and p. 238 ; 1908 and 1909.) The calc-flintas exposed in these quarries present considerable diversity of colour and texture. Microscopic examination shows that the great majority fall naturally into two groups : (1) Grranular, varying in tint from pink to yellowish-brown. (2) Compact and more or less bright green. (1) In the more granular types the pink rocks contain garnet in excess, so that it forms a kind of groundmass ; in the yellowish brown rocks this groundmass is formed of idocrase. In either case the crystals are generally of considerable size and interlock, only a few individuals being partly idiomorphio. In this groundmass are set a great number of small grains of pale-green pyroxene, and it is rare to find a crystal of either idocrase or garnet quite free from them. These small grains are in most cases arranged in rigidly parallel lines, showing the influence of the tension set up anterior to the metamorphism. The phenomenon is identical with the parallel arrangement of the micas through the andalusite in the altered killas already described. The garnet is often anomalous and shows a similar arrangement of the doubly refracting layers to those of the garnet at TreguUan, to the south-west Of Bodmin. The idocrase has little color in thin section, but the double refraction is fairly well marked, though the tint never exceeds pale grey, and is occasionally dark blue. It can be distinguished from the garnet by its more uniform double refraction, lower refractive index and cleavage. At times the grains of pyroxene arranged in lines increase in number till they form a separate band ; in this case the crystals become larger and interlock ; the foliated character is now lost as they form a kind of groundmass, as do the idocrase and garnet in the rest of the rock ; but the pyroxene crystals never attain the size of the two other minerals. In the pink and brown bands epidote and zoisite occur, but are not common, while granular sphene is distinctly rare. Other minerals present in small quantity are more common in the compact green type. (2) The green compact rocks are composed mainly of green pyroxene associated with a subordinate amount of garnet and idocrase, and at times a considerable quantity of a micaceous mineriu occurring in lenticular aggregates and probably secondary. Pale green hornblende, granular sphene, epidote and zoisite are present in small quantity ; while calcite occurs in small but numerous patches. So long as the rock retains a bright green colour there is always a small quantity of garnet or idocrase present, and pyroxene granules generally occur within them. In the greenish grey rocks there is much less green pyroxene ; epidote and Koiaite are more abundant, as are also calcite and the micaceous mineral. In 7^ GEOtOGT OF PADSTOW. addition there is a very fine dark dust spread through the whole rock ; a similar dust occurs in the other rocks already described, but the amount of it is insignificant. In the finely banded rooks composed of alternations of (1) and (2), each band retains its normal composition, though the crystals are necessarily smaller. When one of the laminae consists of a mere film of intensely sheared greenstone, that rock can be at once identified by its composition. The band of mudstone, some six feet thick, seen in the northern quarry, contains at least tO per cent, of free calcite. The more common minerals in it are pyroxene and pale green hornblende, together with fair sized grains of iron ore. In addition there is present a great amount of very fine dark dust, so arranged as to clearly reveal the intense contortion of the rock anterior to its thermo-metamorphism. The fact that it has entirely escaped absorption suggests it is carbonaceous material, possibly true graphite. A second locality is about Trethin ; the rock is exposed in an opening a little east of the farm-house, and specially good specimens may be obtained from the field-walls in this neighbourhood, on this side of the river. The flinty biotite hornfels and the calc-flintas are again well exposed in the section of the river between Helsbury and Trecarne, and the best localities for collecting fresh material are, (1) at and about the old level close to the river opposite Hennon Farm, (2) the bed of the river north-west of the farm, and (3) the quarry just below Trecarne, by the roadside. The pyroxene in the thin altered calcareous layers in these rocks is often so abundant and so white that the rock looks like an alternation of layers of shale and siliceous material. In the specimens dug out of the fields close to the south margin of the granite these rocks are much more crystalline. A typical example is composed mainly of sheets of pyroxene and a smaller quantity of plagioclase felspar. The pyroxene breaks up under crossed nicols to a great number of individua.ls mutually interfering with one another. The felspar patches are in the main optically continuous and contain grains of pyroxene within them. Neither mineral shows any appreciable trace of idiomorphism and the structure is finely granular. Scattered irregularly through the rock are a number of groups of crystals of sphene, in some of which the double wedge form is well defined. In a few rare cases axinite has been met with in these calc-flintas. A large block of this kind was lying by the road-side, west of the strip of alluvium, about a mile to the north-west of St. Neot. Flinty Biotite Hornfels, or Leptynolite. The altered calcareous rocks described above are intimately asso- ciated with, certain hard, often flinty, brown rocks, to wbich. the name leptynolite may be applied. In the type rock, brown mica is always present in considerable quantity and is much in excess of the white mica ; indeed, the latter is often absent altogether. The cross fracture is generally sub-conchoidal, but the rock splits readily along a plane determined by the parallel arrangement of the crystals of brown mica; in fresh specimens these mica crystals impart a distinctly bronzy aspect to the splitting-face. The rocks vary somewhat in composition ; in a few cases they are com- posed almost exclusively of quartz and brown mica; more com- monly a little white mica and a considerable amount of felspar is present. The latter occurs in small grains filled with inclu- sions. It is untwinned, but must contain some lime, as it has a higher refractive index than that of quartz. Many of the crystals are zoned. In other specimens an alkali felspar envelops the quartz grains, and this structure gives the rock its peculiar toughness. The finest specimens are met with in the neighbourhood of Trethin, about a mile Boutb of Camelford. They were obtained from the workings in an old lead mine, and part pf ^he tip-heap is still seen close to the road-side, just, south ALTEKED KILLAS. 73 of the farm-house. Of two specimens selected, the first (6185) is composed of abundant elongated crystals of brown mica, with marked parallel arrangement, set in a matrix composed of small grains of quartz and felspar. The second (6186) contains a fair amount of white mica, as well as of brown, in parallel arrangement. Small grains of quartz are fairly abundant, as are also minute specks of iron oxide grouped in parallel streaks. Felspar is abundant and often occurs m large sheets, enveloping the other constituents. Much of the material taken from the old mine was built into the neighbouring field walls. Good specimens showing minute interbanding with a subordinate amount of calcareous material were obtained from the quarry just below Trecarne, some four miles south of Camelford. Still further south in Lamphill Woods these rocks form little bare scars from which large fragments may be obtained ; they do not appear to differ in com- position from other specimens. In the area south of Wenfordbridge, the flinty biotite hornfels is not so well seen, and in all cases seems intimately interbanded with calcareous material. In this part of the aureole the flinty biotite rocks are close to and practically pass into another group of flinty rocks, which we shall now proceed to describe. Sheared Adinoles associated with the Greenstone. Along a portion of the aureole of metamorphism, the green- stone has been found to be associated with a pale, often flinty, hornfels, characterised by the presence of much alkali felspar. It differs from the flinty biotite-hornfels in containing little brown mica. From their composition and intimate association with the greenstones these pale hornfelses are believed to be adinoles. In most cases the structure now visible in micro- scopic sections of the rocks has resulted from the subsequent thermal action of the granite intrusion, and it is not possible to say what earlier structures may have been present. But on the east side of the De Lank River, where the greenstone occurs near the outer margin of the granite aureole, the associated pale hornfels still partly retains the structure as well as the composi- tion of an adinole. The adinole and the original margin of the greenstone have been torn up into small lenticles, and the junctions are not normal. In a few cases, however, larger compound lenticles can be found in which the original junction is shown, and then the adinole nature of the pale material is obvious. A section of the latter shows traces of the characteristic fine plumose arrangement of the albite crystals ; but a coarser reconstruction may be recognised, starting from a number of centres. Where the adinole was greatly crushed anterior to the heating by the granite, total reconstruction has taken place; this part of the rock now possesses a fine granular structure, and is composed mainly of alkali-felspar and quartz. To the south-east these rocks steadily approach the granite margin and the succeeding outcrops are too far within the metamorphic aureole for any trace of the earlier adinole structure to remain. iNear Blisland adinoles occur both in the bed of the stream and on the north bank above. In the stream they may readily be found, as good specimens are in contact with the small granite veins, which are easily recognised. The other outcrop referred to occurs in the by-road to Lower Polpry. In this case it is impossible to prove that the rook which is distinctly banded is not an altered adinole, because the original junctions have been disturbed, and it might have been next a greenstone at one time Still the aspect of the rock and similar ocQurrences elsewhere suggest that in this case the felspar-bearing 74 GEOtOGY OF PADSTOW. flinty hornfels owes its present structure solely to the effect of the granitic intrusion. The alkali felspar has probably been developed at the expense of the pale micaceous material present in the original sediment, as suggested in the felspathic hornstones around the St. Austell granite.' The alkali-felspar rocks undoubtedly associated with the greenstone are met with again to the north-east of Millpool, on both banks of the stream. A quarry, opened in them for road metal, shows that the tearing up of the rocks is So intense that it is difficult at first to believe that we have here an intrusive sill and its original contact margin. In many cases the two rocks are carded up together so that a banded material is produced built up of alternating films of each. In this case the pale material assumes a somewhat leathery aspect ; but it always retains its compact cross-fracture. A typical specimen is essentially a very fine granulite, containing little biotite and no muscovite. In this ground- mass are set a number of patches of alkali felspar, some of which show carlsbad twins of perthite, while others possess a structure closely resembling that of microcline. Coarser streaks, marking the position of earlier crush-planes, are now built up of larger grains than the granulite ; the chief constituents are quartz and alkali felspar, but a little plagioclase is also present. Close to the farmhouse, nearer the high-road, the actual junction of the old sill and the altered sediment is clearly exposed. Pneumatolitic Changes in the Killas near the Granite. An examination of the killas that occurs at, or close to, the granite-margin shows differences due not merely to original diversity in composition, hut also to the varying extent to which the rocks have been invaded by gases, tending to produce the special type of alteration known as tourmalinisation. The extent of this alteration may be small or great, and it has become clear that comple'te tourmalinisation is not absolutely contempo- raneous with the contact-alteration due to the intrusion, but distinctly later than it. The actual contact rock is exposed at the foot of the small waterfall in the stream below Row, near St. Breward. A section (4730) shows that the killas is a rather decomposed, fine, andalu- site-schist, in which the white micas are of good size. The parallel arrangement of this mica, seen in the rocks further from the actual contact, is still preserved to some extent, but the development of crystals at a high angle to the foliation planes is a marked feature of the structure, especially in the case of the red-brown mica. In the micaceous groundmass are set patches of decomposed andalusite and the texture of the rock, so far, is that of a typical fine andalusite-mica-schist near the granite. But in addition there are present a number of grains and small crystals of tourmaline tending to occur in groups. These do not in any way interfere witE the general structure of the rock, neither do they replace any pre-existing mineral, such as brown-mica, which nowhere shows any trace of pneumatolitic alteration. The tourmaline thus owes its existence to the addi- tion of a small quanti1;y of a special gas to the water vapour contained in the hydrated material of the killas. The exact stage at which all the tourmaline in these rocks is formed is not always so clear; in specimens collected in and near the stream at Bury Castle (7347), to the north-east of Car- dinham, the amount of tourma-line present is much greater, and ' Flett, in ' Geology of the Country around Bodmin and St. Austell,' (Mem, Geol, Surv.), 1909, p. 100, JPNETTMATOLITIC CHANGES. 75 some, at least, of this is replacing part of the earlier formed contact minerals. The explanation of the difflculty is suggested in a paper ' On some Brecciated Stanniferous Veinstones,'^ where it is shown that more than one epoch of invasion by gases can be detected. In the case of the rocks at Bury Castle, there are probably both contemporaneous and subsequently formed tour- maline. That the more complete phases of tourmalinisation are of slightly later date than the contact action is best shown by selecting a partially tourmalinised specimen further from the granite, where the adjacent rock contains no tourmaline. The specimen from a small quarry near Millpool described on page 67, shows perfectly the phenomenon of the replacement of contact minerals. This specimen is taken from the margin of a minute fissure up which gases have ascended and deposited schorl and a small quantity of ore, said to be tin ore. Away from the crack the rock is a normal fine andalusite rock ; but in the specimen close to the crack the andalusite has been replaced by white mica (? gxeisen-action) and the whole of the brown mica has been replaced by tourmaline, which is massed on the margin of the altered andalusite exactly in the position so commonly assumed by the red-brown mica in the typical contact-altered andalusite rocks.' When completely tourmalinised the killas is in most cases composed of tourmaline and quartz, and specimens lying loose on the ground appear quite black. Rocks of this type are met with close to the granite at Pendrift, half a mile north of Blisland, and extend some distance from the granite margin. This complete tourmalinisation of the killas is distinctly rare in the portion of the aureole round the Bodmin granite here dealt with; it has been met with again to the east of Warleggon, but only in small patches. In the great open-work on the margin of the granite near Treveddoe the killas at the actual edge of the granite is too decomposed for accurate examina- tion ; specimens (7352-3) show clearly that the rocks a little further from the granite are often more markedly tourmalinised than those nearer. This seems due to the more altered specimens being quite close to, or traversed by a small fissure along which gases have ascended, and in most cases no great amount of tourmaline is produced by gases derived from the edge of the granite as it cooled. On the east side of the Camel, to the north of St. Breward, where the granite margin comes almost to the river side, there are numerous exposures of the killas both in contact with the granite and at distances ranging up to 50 yards from it. The rocks, are of much the same nature as. those described above. Altered Greenstone. One or more sills of altered greenstone occur within the metamorphic aureole. To the south-west of the granite a fairly continuous outcrop can be traced on the ground between Penrose Burden near St. Tudy, and Millpool, to the north of Cardinham, terminating at the granite margin. The same sill is again met with in each of the_ streams further east, but only in isolated . patches, too small to be shown on a one-inch map. To the north of St. Neot there is another but shorter continuous outcrop on the north side of the elvan ; the greenstone in this part of its course is often well preserved, especially on the east side of the river bank. Along the western margin of the granite, to the north of Penrose Burden, the discontinuity of the outcrops is equally marked. A clear instance of this is seen on the west bank of the Camel, to the south of Camelford and opposite the ' Plett,iM 'Summary of Progress' for 1902 {Mem. Geol. Surv.), 1903, p. 154. * ' Geology of the Land's End District,' {Mem. Gepl. SurvO, 1907,_P1._I, fig. 5. 76 GEOLOGY OF PADSTOW. church. Here a lenticle of greenstone occurs, ten feet thick in the centre and tapering away at both ends/ The mapping o± the larger and coarser masses at Michaelstow suggests that the sepa- ration of the outcrops is due to powerful earth movements, and is not original. Over much of the area these movements have broken down tlie ojiginal components of the rock and produced a milled structure marked hy a pronounced parallel arrangement of the con- stituents. This parallel structure is in most cases preserved m the recrystallized rock, when subsequently metamorphosed by the granite ; so that the greenstone is now a very fine-grained aggregate of actinolite needles, minute grains of felspar, and usually some quartz, together with iron ores in a fine state of subdivision, or their replacement products containing titanic acid, such as leucoxene or sphene. These constituents are naturally smaller when the outcrop is further away from the granite than when close to it; as the crystallization advances, the component minerals increase in size and the leucoxene disappears, being replaced by granular sphene. In no case has the greenstone been found close enough to the granite for brown hornblende to be developed. The greenstone has a coarse texture, and the original structure is more or less preserved in the larger masses about Michaelstow, De Lank Water, and Blisland. The larger patches of actinolite or epidote present in these rocks do not result merely from recrystallisation, but represent approximately the position of crystals of augite in the original rock. Much of the sill had an equally coarse structure originally, but this was obliterated by crushing; where the original sill was thinner it was naturally finer in texture, but now it is impossible to deter- mine to what extent the more or less persistent structure is original or due to the crushing of an originally coarse rock. The coarser rocks have always a margin of fine schistose material, as may be seen in the smaller mass to the east of Helsbury Castle. A typical example of a very fine-grained sill occurs in the northern of the two quarries at the ford, about a mile east of Camelford. This (6151) is a rather dark, hard but somewhat fissile rock, the splitting face of which is partly coated with brown mica. It is composed of very small crystals and grains of actinolite, felspar and iron ore together with a small quantity of brown mica. The minerals are on the whole evenly distributed and the rock has a flat foliation. The actinolite is largely arranged parallel to the foliation, but it is locally aggregated in thin patches, within which the individual crystals are not parallel. The iron ores are somewhat elongated. .Some of the felspar has a higher refractive index than balsam, and is probably oligoclase. The rook was greatly milled or crushed and the parallel structure then set up has been intensified by the granite alteration, since the rock is some distance from the ' granite the reconstructed material has naturally a fine texture or grain. A good example of the slightly coarser variety occurs in the small quarry on the east side of the road from Keybridge to De Lank. In this the white specks of felspar ars easily seen in the -dark ground-mass, and the rock is far less fissile than the last. It is composed (4745) of small crystals of felspar, much of which appears to be original and still shows twinning, and is largely oligoclase ; irregular shaped patches of actinolite with feathery ends replacing the torn out pyroxene, which has entirely disappeared ; iron ore slig;htly broken down, and apatite practically unaltered. The patches of actinolite are arranged in approximately parallel layers and impart the slight fissility to the rock. ' It is impossible to show these small masses on the one-inch map ; but they are marked on the six-inch maps deposited in the Survey offices. EPIDIORITE. 77 The coarse type which may be described as a fine epidiorite, occurs at several localities. The largest masses are met with near Michaelstow and are shown on the map. The coarse rock is seen again in a low crag some 200 yards south of the De Lank River. This (4789) retains practically its original texture, which was that of a subophitio dolerite. The felspars are largely preserved, as are also the iron ores and the apatite crystals, but the original augite is entirely replaced by either actinolite or epidote. On the east aide of the De Lank Water, about a quarter of a mile above Keybridge, is a good exposiire of a basic sill, apparently repeated by folding. The more southerly outcrop is much sheared and quite thin ; probably part only of the sill is seen here. The more northerly is free from shearing in the centre, but much crushed on its edges and completely intermixed with the curious white material resembling adinole (described on page 73). Between the two outcrops is a considerable thickness of hard rock, having a parallel structure ; its compactness is suggestive of the original contact action of the sill before folding. 1B99S» F 2 78 CHAPTER VII. TERTIARY AND DRIFT. Teetiaey. Nothing definiie is known as to the Tertiary geology of this part of Cornwall ; but we find surface features evidently of great antiquity and pointing to conditions very difEerent from those now prevailing. Though many of the contours of Bodmin Moor may belong to far earlier times, the oldest feature which we can date with any certainty is a gently sloping plateau, with an upper limit of 430 feet above the sea, which is well seen "near Trebarwith. This corresponds so closely with the early Pliocene shore-line already recognised in West Cornwall,^ that there can be little doubt as to their being contemporaneous. We will, therefore, take this plane of marine denudation as a datum from which to measure. Bodmin Moor exhibits some very singular features above and older than the Pliocene plateau. Instead of the granite hills showing a regular valley-system of subaerial origin, they are cut into by two wide and conspicuous shelves or plateaus at the heights, respectively, of about 750 and nearly 1,000 feet above sea level. ^ The higher of these plateaus is seen only on the ground about the heads of the streams, and generally forms part of the water- shed; it lies completely a'bove the mflre recently deepened valleys to be described later on. The best general view of this feature is obtained from a point on Davidstow Moor to the north of Crowdy Marsh. As viewed from this point the conformation of the country suggests that this platform once covered an exten- sive area, and must be of considerable antiquity; the hollows in it are shallow but very wide, testifying also to its great age. An examination of the northern and north-eastern part of the granite area further suggests that many of the high tors were islands rising from the sea at the time the rock-platform was being cut ; indeed, the form of the ground must have somewhat resembled that of the Scilly Isles at the present day. This kind of scenery may be best observed in the neighbourhood of Bol- ventor. Further south a trace of the old sea-margin is visible at the north-west foot of Browngelly Downs. On Davidstow Moor much of the flat ground i.s covered with ' head,' or material decomposed approximately in situ, this head at times being as much as ten feet thick. Some pits sunk in this to the underlying rock failed to reveal the presence of any remnants of gravel or beach-deposit on the platform, consequently, there is no means of fixing its true age. The 750-foot platform is easily recognisable in the area about Camelford, and a good general view of it is obtained from the private road leading to Parkwalls, which lies about a mile east- ' Reid, Nature, 1886, p. 341 ; and ' Pliocene Deposits of Britain,' (^Mem. Oeol. Surv.), 1890, chap. v. ' Barrow, Quart. Journ. Oeol. Soc, vol. Ixiv, 1908, pp. 384-400, south-east of the town. Its great extension can be readily recog- nised. The portion north of Camelford forms a hay, the south-eastern side of which is still fairly well defined by the rising granite ; the north-west by the high ground north of Dela- bole. The floor of this old bay is cut through by a gorge of such recent age that one can look across the flat without being a,w^are of the existence of the valley. Another good general view of the 750-foot platform may be obtained from the footpath a little, north-east of St. Breward, looking toward the south-east. The broad flat feature, here entirely within the granite area, stands out clearly; but if the ground be traversed it is found to be intersected by several later valleys of considerable breadth, marking the large amount of denudation that has taken place in this part of the area since the formation of the plateau. No trace of any deposit has yet been found that could establish its age. The cutting of this platform led to a great modification of the previously existing drainage. The old Pliocene coast-line is now represented by a steep bluff rising out of a plain which has its upper edge at about 430 feet above the sea. Where the slates are soft this bluff may merge almost imperceptibly into the plain below and into the slope above ; but it is clearly recognisable at two points in our area, where at the right elevation hard rocks faced the waves. On our southern border the Pliocene cliff forms a conspicuous feature in the Staddon Grit, when the ridge is viewed from the neighbourhood of Padstow or Newquay. This grit, it will be remembered, makes a conspicuous ridge of high ground stretch- ing from Bodmin nearly to the sea; but though the hard strata are continuous, the ridge is not. About three miles before the coast is reached the ridge is cut off at right angles by a sharp bluff running north and south, everything west of this point lying below 430 feet, soft shale and hard grit both having been planed down to a nearly uniform level. Though the old cliff-line is so conspicuous where it has faced the waves, it becomes very obscure when we attempt to trace it on the lee-side of the high ridge. It is perhaps traceable on the north side of St. Breock Downs, but east of that the numerous tributaries of the Camel have so trenched the slopes that the Pliocene bluff is no longer recognisable. Perhaps the coast-line once trended northward, across the present lowland, to Port Isaac Bay, for at Bounds Cliff there is a sudden rise in the height of the land, unexplained by any change in the character of the strata. The Pliocene cliff reappears three miles to the north as a well- marked feature, running in from the coast at Tregardock, and then trending almost parallel to the modern cliff to Trebarwith and on to Boscastle. At Trenale, a mile east of Tintagel (in Sheet 322), on the shelf just below this old cliff, there is found a small patch of well-rounded shingle of vein-quartz, and the farmer states that he has found sea-shells when digging in it. No open section is now visible, and a small pit we dug for the purpose of testing this statement produced no shells, though it yielded undoxibted beach-pebbles, under eight feet of head. 80 GEOLOGY OF PADSTOW. The occurrence of a Pliocene shore-line at the same elevation at the Land's End and near Tintagel, sixty miles apart, seems to imply that this great change of sea-level has taken place without any tilting perceptible in that distance. A probable inland continuation of the Pliocene-platform occurs on the east bank of the Camel to the south-west of Helland and on the south bank of the Clerkenwater. At Helland itself, but at a slightly higher level, is a sloping flat-based feature, suggest- ing an inland valley opening out on to the platform. An abnormally large patch of alluvium occurs on one side of the De Lank Water, about a mile above its junction with the Camel. The origin of this seemed at first inexplicable, but its height above sea level suggests it was a small bay-like extension of the Pliocene platform, now left isolated by more recent denudation. A curious phenomenon connected with the higher platforms described is the occurrence of wide gently-sloping depressions at the upper margins of each of them. These flats are now covered with peat, of quite recent formation ; but the flats themselves are far older and are little more recent than the platforms. At the north end of the granite the marshes of Crowdy and Casparpool are on the 1,000-foot platform, while Stannon Marsh occurs about the head of a bay of the 750-foot.^ These marshes are interesting as affording the only chance of finding any remnant of the gravels that once lay on these plat- forms. IN'o fossils are likely to be found, as they would long since have been dissolved by the peaty water. Excavations for stream-tin or wolfram may at any time expose such deposits; but at the time we write, all these hollows are occupied by undrained marshes. From the Pliocene clifi: the plateau slopes gently downwards towards the present coast, but close to the coast the grassy slope usually becomes much steeper. We cannot yet date this inter- mediate slope, which is a common feature in various parts of Cornwall; it may either represent a Newer Pliocene shore-line, or it may be due merely to the rounding off of the cliff-edge which took place under an Arctic climate ; at any rate it is not a feature which is being developed under present orographic and climatic conditions. Raised Beach. Below the steep slope just described comes the vertical cliff, at the bottom of which here and there is preserved a narrow rocky shelf or notch, rising a few feet above high-water mark and c6vered with well-rounded beach shingle. This shingle is the well-known ' raised-beach,' which fringes all our southern and south-western coasts. In this area the attacks of the waves have generally been sufficent to destroy the narrow ledge, but it is well preserved in the estuary of the Camel. If we compare this Pleistocene raised-beach feature with the three higher platforms ' Similar marshes occurring on the Pliocene platform are described in the explanation of sheets 346 and 347. Photographs of two of the marshes are included in the Survey Collection of photographs (Nos. 494, 495). RAISED BEACH. 81 above described, we are at once struck by its comparative insig- nificance, for in Cornwall it seldom appears to be more than a few yards in widtli. In reality it is not so unimportant, for we must bear in mind that what we see is only the upper edge of the raised-beach platform, the rest of it is below the sea. Judging from the submarine contours this platform may be of considerable extent, but it seems impossible in this area to disentangle the features cut by the sea during the successive rises and falls that marked the post-Pliocene period. On entering the district from the south we meet with vertical cliffs of grit and slate which waste too rapidly for the raised-beach platform to be preserved, and it is not till Constan- tino Bay and Trevone are reached that we come across a rocky platform rising above high-water mark. This shelf, however, has no beach-deposits lying on it, unless such are hidden under the sand-dunes. On rounding Stepper Point into Padstow Harbour we at once perceive a narrow rocky shelf about five or ten feet above high- water and extending from Hawker's Cove to Harbour Cove; just north of St. Saviour's Point a small patch of beach-shingle is preserved, resting on the shelf and overlaid by rubbly ' head.' The south bank of the Camel also shows traces of this shelf at Padstow and Sunnycorner. In the lateral valley leading to Little Petherick, under Trevorrick, a strip of flat land occurs a few feet above the water level, and shows a low cliff of coarse boulder- gravel consisting of subangular vein-quartz and grit blocks, the latter evidently derived from the Staddon Grit several miles away at the head of the valley. This deposit suggests perhaps flood- action and river-ice ; but we cannot say whether it is exactly con- temporaneous with the raised-beach, or should be referred to the later head. Re-entering the main valley we find two miles higher up, below Penquean, another patch of boulder-gravel about 5 feet above high-water level. It is a coarse rounded gravel, of vein-quartz, granite, elvan, and metamorphic rocks, derived from the outskirts of Bodmin Moor, ten miles away in a direct line and twenty miles up the valley. This gravel seems to suggest the agency of floating ice, for it is far too coarse to be transported down so gentle a slope, even in times of flood. Possibly, however, Penquean was then the head of the estuary ai^d the gravel is a delta-deposit laid down where a rapid river entered the tidal estuary. If this were so, it suggests that the valley above has been greatly deepened in later times. In favour of this latter view we can point to the occurrence of a single outlier of similar coarse boulder-gravel, forming a terrace 40 feet above the adjoining marsh, in the fork of two streams south-east of St. Breock church, and exposed in the road-cutting. We have seen no other terrace-gravel at a similar height in the Camel valley. On the right bank of the Camel we do not meet with the raised- beach feature east of Cant Hill. It occurs under Rock golf links, and at Trebetherick Point it is very conspicuous; the coast between here and Pentire is probably the best place in the district for the study of the relations of these Pleistocene deposits : we 82 GEOLOGY OF tADSTOW. will therefore describe the sections in some detail. It will be noticed that far-travelled boulders occur, and these should be compared with those described above as occurring at Penquean. A general view of Trebetherick clifE is shown in Plate I., at the end of this Memoir. At and south of Trebetherick Point cliffs, twenty feet high, are found. The bottom ten feet is solid rock; the top consists of sandy and pebbly beds. Lying immediately upon the flat top of the ten-foot cliff a thin layer of rounded pebbles appears at intervals. This layer is covered by sandy and clayey bands for another five or six feet, upon these lies a bed of boulders, one to six feet thick, beneath recent blown sands. The beach-material consists of rounded pebbles of quartz, grit, slate, and flint. Head immediately overlies the raised-beach, but at the first small cove north of Trebetherick Lane a change takes place. On top of the slate cliff a little head is overlain by six feet of beach containing rounded pebbles; then a second layer of head, another boulder bed of large rounded stones, and modern blown sands on top. As the coast is followed the lower layer of beach becomes sandy, while the head between the two pebbly bands is very constant. The lower pebble bed soon disappears and the head increases in thickness, while the raised beach platform is everywhere well exposed. As the open sea is approached the pebbles become much larger. By the third wall along the cliff-top the following section is seen : — Recent blown sands. Thin layer of quartz fragments. Sandy band — two feet. Head, purple and green slate fragments — one foot. Beach stones — one foot. Solid rock. At Trebetherick Point the cliff is nearly, forty feet high and is composed mainly of cemented sands, which are seen in section near a cart road to the beach. The alternating hard and soft layers have been affected differently by weathering, the hard layers being separated from one another by the undercutting of the softer material. The beds are sands cemented with calcareous matter and contain shells of limpet and crab carapaces. But besides the sand grains water-worn frag- ments of purple and green slate lying flat are seen in the cliff. Cemented sand-reefs identical with the material forming these cliffs are found at sea level in Daymer Bay and upon this the sub- marine forest is rooted. At the seaward end of the cart-road a ten foot band of rounded stones, of quartz, greenstone, elvan, schorl-rock, quartzite, flint, hard grit, and pebbles of consolidated sand derived from the stratum below, runs northwards for a distance of fifty feet. It tapers off at each end. Near the top of the slate cliff a small patch of head underlies the boulders, and the boulders rest on the cemented, sand. The order of deposition would therefore appear to be : -T-Eaised-beach ; sandy, marly, and cenaented sand ; head • boulder-beds; recent blown sands. DRIFT. 83 RivEK Terraces. Hiver terraces, perhaps of the age of the raised-beach just described, are conspicuous in various parts of the Camel valley between Wadebridge and Oatlands Wood, sometimes forming con- tinuous shelves. They gradually rise inland to over 100 feet above the sea. In the fork of the valleys at Polmorla, south-west of Wadebridge, on a shelf at sixty feet (forty feet above the adjoin- ing marsh) there is seen in the road-cutting a patch of coarse boulder-gravel. This gravel consists of large rounded masses of quartzite, derived from St. Breock Downs, two miles away. No such boulders now travel down the valley, even during the most violent floods. That the gently inclined upper portions of these moorland valleys once extended much further seaward is proved not only by the terraces of the Camel but by the occurrence of a patch of gravel above Treganna Wood, about three-quarters of a mile to the north of St. Neot. It is oval, about 300 yards long by about 150 yards broad, and lies about 180 feet above the present bed of the river. The gravel is largely composed of well- rounded granite pebbles, some of considerable size, and a con- siderable amount of stream-tin was obtained from its base. It obviously formed part of the bed of the St. Neot Eiver, when the latter flowed at a much, higher level, and! further, it proves the former southern extension of the slow fall that still characterises the tributaries of the Fowey within the granite area. Not only has the gravel been swept away, but the rock beneath has been excavated to a depth steadily increasing as we come south. This isolated patch of gravel differs from the material well within the granite area in two points ; first, it has not undergone the complete re-sorting that characterises the tin-bearing deposits in the granite-valleys, and, secondly, there is no trace of the newer fine deposits above the tin-gravel. Thus the isolation of the gravel patch . was anterior both to the newer conditions of deposition and the older one of excessive fioods. Further, its height above sea-level (600 feet), as well as its height above the present river bed (180 feet), suggests that it may be of Pliocene age, but this cannqt be proved. An examination of the sur- rounding ground shows that great denudation has taken place since its formation. The deposit itself rests on killas, but for the granite pebbles to have reached it there must have been a continuous connection between the site of the deposit and the granite. This connection is now severed by a branch valley, almost at right angles to the main one, nearly 100 feet deep, almost a mile long, and cut mainly in killas. This branch valley could not have existed when the gravel was deposited, for in that case no granite pebbles could possibly have reached it. On the opposite side of the river, but at a lower level are some later gravels, also tin -bearing, that at their highest point lie much above the present river level. ' Other deposits probably of a date equally remote are described in the next section, for in the upper parts of the valleys it is 84 GEOLOGY OF PADSTOW. generally quite impossible to distinguish between the ancient and the modern alluvia,_and they must be mapped as one. As the distribution and extent of the alluvial deposits depend largely on the form of the surface, it is desirable to give a brief account of the recent history of the valleys in this area. Many of the valleys, above ihe modern silted-up estuaries, may be divided into two parts, one characterised by a rapid fall, the other by an extremely slow fall; in the former the valley deposits are usually thin, in the latter they are always thick. This difference is directly due to the Pliocene uplift, which led to a rapid deepening and cutting back of all the larger valleys. In the case of the Camel, the main river has deepened its bed, some 22 miles up the valley, as far back as Slaughterbridge, above Camelford. At the point where this deepening ceases the river has a specially rapid fall ; immediately above this point the fall is quite slow. Moreover, the banks are always steep in the deepened portion, but have a much lower slope above the rapids. These features are not confined to the main river Camel, all its larger branches show similar phenomena. The best illustration of a very rapid fall, in the section of a stream where the cutting back ends, occurs beneath the granite crags of the Devil's Jump, some three miles south of Camelford. In this case the cutting back has led to the capture of the water of a much larger stream and the increased flow has greatly aided the recent deepening of the bed of the capturing stream. The De Lank Water forms a rapid, almost a waterfall, at the granite works, but above this the fall is much slower, and we enter the section characterised by thicker alluvial deposits. Similar features are met with in the valleye of the Fowey and its tributaries ; the point marking the cessation of the cutting back being always characterised by a specially rapid fall, as may be seen east and west of Warleggon, and, again, a little above St. Neot. The most striking illustration of a gorge-like, over- deepened valley occurs to the south-west of Cardinham, close to the southern edge of the map, where the valley is cut in the softer killas outside the aureole of alteration. All the upper portions of the tributaries of' the Fowey, where they flow through the granite area, are characterised by an extremely slow fall, and in specially wet seasons they flood the alluvial flats which they traverse. Similar, but much shorter, sharp falls are occasionally met with in the tributary streams at about 800 feet above sea-level. These suggest that a rapid emergence took place prior to the period of comparative stability, during which the erosion of the T 50-foot platform previously described was accomplished. The slow fall of most of the streams above the over-deepened portion is as well marked in killas as in granite areas, but there are certain features that are best seen in valleys cut in the granite. Of these the most important is the occasional great breadth of their flat base when compared with the size of the streams flowing through them. Indeed, in extreme cases broad, flat hollows occur in which there is no stream at all in dry weather, and only STREAM TIN. 85 a very small one in heavy rains. The abnormally broad portions of the granite valleys are primarily due to the naturally hard granite being converted to a soft material by kaolinisation. This type of alteration varies much in extent ; in its least pronounced form the granite so afEected can easily be washed down by a moderately strong jet of water ; when the kaolinisation is com- plete, the whole mass becomes quick, when thoroughly wetted, and flows almost like water. A stream flowing through this material rapidly cuts down to a local base level, and then by oscillation widens the base of the hollow through which it flows; or even rain falling on a soil free from vegetation will have the same effect. The connection between the amount of superficial deposits and the history of the valleys now becomes obvious ; above the over- deepened portions there is a much greater amount of sand and gravel, as their streams have not the power to keep them clear; further, there must be a specially large amount in the exception- ally broad parts of the granite valleys, where that rock has been kaolinised. Lower down the valleys, where they are over- deepened, the alluvial material has been completely resorted, much of it washed right away, and the very rock-base, on which it formerly rested, has been cut into. Stream Tin and Wolfram Deposits. So much light has been thrown on the superficial deposits by the workings for stream tin on Bodmin Moor, that to avoid repetition the two may be described together. In strips of varying width these old workings cover mile after mile of country, and the industry for 1,600 years has been carried on in situations that in many cases have no parallel in the more westerly granite masses of Cornwall. The exceptional extent of these workings is due to a combination of two causes; first, to their height above sea-level,^ and secondly, to their occurrence mainly above the over-deepened portions of the valleys, where the old deposits have been least disturbed. Owing to the height of the area above sea-level the granite has been exposed to decomposition for an exceptionally long period, during which tin and wolfram ore has been slowly set free from veins and innumerable small strings, and possibly from the granite itself, to accumulate at the foot of the slopes. Part of this accumulation has come within reach of the streams within the valleys, when it has been rolled and assorted, the finer and lighter material being in part washed away, the heavier ore and the associated gravel left. Part, how- ever, still remains at the foot of the slopes as unassorted wash. In this the ore is mixed with a larger amount of granite debris = than in the material actually in the stream bed. This wash is not confined to the sides of M'ell-marked valleys : recent trials show that at high levels it has remained in almost every slack and hollow in which it could lodge. It probably maintained this position owing to the fact that it was either frozen or covered by snow. Sections of the deposits in the larger valleys show that ■ Barrow, Quart. Joum. Geol. Soc, vol. Ixiv, 1908, pp. 384-400. 86 GEOLOGY OF PADSTOW. there was a period of exceptionally heavy flooding, during which the force of the running water was sufficient to van or assort the deposits at the bottom of the valley. As a result there was left only the coarser material of the gravel, associated with the heavy ores of tin and wolfram. Almost all the fine sand was washed away. Moreover, a specially deep though often narrow channel was cut in the comparatively fiat floor of the old valley, and most of the tin-bearing gravel was washed into this, rarely covering much of the main floor. The force of the flood water was such that the wash on the slopes could hardly have largely escaped unless it had been frozen or covered with snow and thus enabled to resist such powerful denudation. The gravel just described has since been covered by later deposits, mostly of a fine nature, though locally a little fine gravel was mixed with it. Finally, the whole has been covered with a sheet of peat varying in thickness, but rarely less than 5 -feet, and in some cases attaining as much as 18 feet. It is thiis seen that the period of washing or vanning the superficial deposits was succeeded by the more modern conditions, in which the streams cannot keep clear the main floor, but are actually depositing fine sand and mud over the basal gravels. In some cases this newer deposit has attained a thickness of over 25 feet. The salient features of these deposits in a broad-based section may be best made clear by the diagrammatic section given below (Fig. 4) : — Fig. 4. — Section showing the relation of the Stream-tin and Alluvial deposits to the Ancient Valley. 'd^\'/A (a) The sides of the valley and the ' shelf,' or true base, are formed of granite ; but if the valley be broad the true base is Usually found to be more or less kaolinised. The case is aptly put by the old streamers who say " when the hills are far apart (valley broad) the shelf is china-clay ; when the hills come together (valley narrow) the shelf is firm granite.!' In the Bodmin Moor granite the sides of these valleys are never appreciably kaolinised. (J) The oldest deposit is the tin-bearing gravel in the deeper channel. It was this channel that the streamers followed up and in many cases they never left it to explore the material on either side of it. (c) Next in age is the true ' wash ' or unassorted and more or less angular material that lies at thqJEoot of the slopes ; sometimes almost extending to the edge of the old channel ; at others ending at a considerable distance from it. In the former case the streamers tested it, and if sufficiently rich followed it so long as it could be made to pay. But, in the second case, there is a considerable breadth of material containing no tin before the wash is reached, and when this occurred the streamers rarely crossed the broad belt of barren ground, but went on up the old deep channel. The wash in these cases has rarely been turned over, and it forms a considerable portion of the whole deposit. The conditions under which a considerable amount of tin may be present in the wash may be briefly summarised as follows. Where the side of the valley has a low slope it ALLTJVIAL DEP6sITS. 87 is generally traversed by veins, and small threads which shed a fair amount of tin into the wash. If the side be steep and the granite solid there is little prospect of any tin. The further the low slope continues the larger the collecting surface from which the ' shed-tin ' can be gathered and brought down into the valley. When examining an unbroken surface of the ground this deposit often appears to be newer than the peat, because a small amount of wash is still forming and is really newer ; it is, however, a mere skin, the bulk of the wash is seen to be of the age shown when a moderately deep excavation is made. It was peculiarly well shown in some recent trials at OoUiford Downs, some two and a half miles north of St. Neot. (d) In its simplest form this consists of fine sand washed down after the period of frost ended and when the floods were no longer strong enough to sweep sand out of the gently inclined parts of the valleys. A period of deposition in such valleys followed the period of strong erosion. These deposits are sometimes more complicated, and specially so if the valley in which they occur is both wide and deep. In 1847 S. R. Pattison published an a'ccount of some stream tin workings in the Fowey Talley and gave a section i f the exposed ground. The peat here was 1 4 feet thick and rested on three feet of . the typical fine sand. Below this, however, came another peat bed, five feet thick, containing wood, hazel nuts and antlers of deer. Under this came the main tin-bearing gravel and sand about seven feet thick. Below this is still another peat bed, very black and hard and containing many trees. This bed, five feet thick, rSsted practically on the granite shelf, but some tin was found beneath it. The trees were so numerous that they were cut up and used by the miners for fuel, and the author states that piles of this wood could be seen stacked at Bolventor. The heads of the trees were in full vigour and lay all pointing in one direction as if blown down by a storm.' The blackness and hardness of the lowest peat, and the fact that it occurs beneath the main tin-bearing bed, are suggestive of its being older than the period of extreme cold and flooding ; if so it is of much greater antiquity than the peat usually met with by the stream workers. (e) The peat at the top of these deposits was so far as known never absent before the stream-workings were begun, arid as the Fowey section shows it was of considerable thickness. Since it was turned over, burnt, or largely washed away, a new growth of peat has taken place in many of the valleys, and at times this has so covered the disturbed ground that only an expert can tell that the streamers have worked the ground. This growth never exceeds a foot or two and is always woolly, not compact ; this is of importance, as recently some ground was declared to have been worked, but by cutting open the peat this was found to be eight feet thick and solid, showing the statement was incorrect. The state of the peat thus affords an easy means of testing if the ground has been turned over or not. The gravel just described forms a kind of link with the alluvia gf comparatively modern age, that occur in the recently deepened valleys right down to and helow sea-level.^ The amount of gravel present depends mainly on the fall of the rivers, and is least when the fall is greatest. Stream tin has always heen found to some extent beneath the gravels, and there is no doubt that the whole of the Alluvial deposits in the recently deepened portions of the Camel and the Fowey have been turned over for tin-ore almost down to sea-level. This could easily be done, for in no part of their course is the slope so small as to put any difficulty in the way of drainage, a trouble that often occurred in the slow-falling section of the streams. The original thickness of the river deposits before they were turned over is unknown ; apparently, , the. sections were considered of little interest, and no record of 1 Trans.. Royal GeoLSoe. Cornwall, vol. vii, 1847, p. 34. ;, ^ The seaward ends of these have still more recently been submerged ; but they were deepened first. 88 GEOLOGY or PADSTOW. them appears to have been kept, though traces of the old workings can be still recognised in many places. Below the sea-level m the Camel valley the stream-tin has not yet been worked. Peat. In the killas area peat is uncommon, but within the granite area every valley-bottom was once covered with a sheet of pea,t, such as already described. Of this, considerable areas still remain undestroyed by the stream-tinners, and in many cases it is extensively cut for fuel. Hill peat is rather restricted in occurrence, and is distinctly uncommon on the west side of the granite area. It forms a rather extensive tract on the west side of Dozmary Pool, and on the flat ground to the north of the road in the Bolventor area. This peat is usually thin, but harder than the hollow-peat. On account of its thinness it is cut almost horizontally, not vertically, and by this means longer pieces are obtained. An interesting point has been proved by this deposit. Both near Dozmary Pool and on Pridacoombe Downs a great number of flints of Neolithic age have been met with at the base of the peat, but never within it. Accompanying the worked specimens are some perfectly rounded pebbles, showing the material had been gathered from the sea-shore. It is clear that no peaty growth existed here when these flints were worked, and that no denudation has taken place over large spaces of flat ground, even on the hill tops, since Neolithic times. The famous Dozmary Pool just mentioned, owes its existence solely to the peat. This began growing in the valley below, and gradually crept up the hill side, attaining a thickness of six feet at the west side of the flatfish hollow in which the Pool lies. The peat shut in the water collected in this hollow, and thus formed this supposed fathomless pool. One of the stream-tinners being short of water for streaming, cut a trench through the peat and almost drained the pool, thus destroying a cherished tradition. Sand-banks and Dunes. The estuary of the Camel is much choked by sand-banks, largely due to the waste material washed down from the stream- tin works in days when no attempt was made to preserve the har- bours. But besides these deposits of granitic and vein debris there are extensive banks of sand washed in from the sea, and these contain a large amount of comminuted shells, especially at Doom Bar (or Dunbar). So valuable was this sand for manure that there was formerly an extensive trade in it, estimated in 1836 at about 100,000 tons per annum. ^ Now this trade has nearly ceased. Wherever in Cornwall we find a foreshore of sand laid bare by the tide, there is a tendency for the sand — especially flaky shell- sand — ^to drift before the wind and accumulate above high-water mark as hillocks of blown sand or ' towans ' as they are locally ' De la Beche ' Report,' pp. 479, 480, 520. The sand of Doom Bar appears partly to rest on a ' submerged forest.' COAST SCENERY. 89 called. These sand-dunes may travel to any height above sea- level, if the slope is gentle. At Constantine the sand drifts right across the isthmus into Harlyn Bay, where it has overwhelmed an ancient British cemetery, It is beyond our province to deal with archaeology ; but it is worth while to draw attention to the value of such ancient burials, or of kitchen middens, for dating the gradual advance of the dunes. The graves at Harlyn are partly dug in blown sand, but they have been overwhelmed by a much thicker sheet of similar material. Another extensive area of sand-dune forms the golf links opposite Padstow, the sand having drifted in before the prevalent wind from the extensive fiats laid bare in Padstow Harbour. Coast Scenery. The coast near Padstow yields a striking illustration of the connexion between scenery and geology, for all the headlands are formed by hard igneous rocks and all the bays by the softer slates between. Thus the most beautiful parts are those in which the geology is most varied. We find a line of straight cliff, with a foreshore only here and there accessible at low tide, stretching from Trenance to the sharp point of Park Head, where a small mass of greenstone resists the waves. North of Park Head the coast-line and slaty cliffs fall away and are only broken by three small valleys till we reach the sand-dunes of Constantine Bay. Then we ai'rive at Trevose Head, one of the boldest headlands in Cornwall, and continued seaward in the inaccessible QuieS. Trevose Head is composed of folds of hard and very tough green- stone, alternating with slate, which is often changed into masses of white chalky-looking adinole. Behind the shelter of Trevose Head comes Polventon or Mother Ivey's Bay, a shallow bay of slate between two headlands of greenstone, the northern of which is shown in the accompanying illustration (Fig. 1, p; 19). It will be seen that Merope Rocks stretch far beyond the general coast- line and are gradually being cut up into isolated stacks surrounded by sea. Cataclews Point and Porthmissen Point are less prominent, for they are formed of a softer type of greenstone, harder than the slate it is true, but easily cut by the chisel. Stepper Point again is a headland of hard tough greenstone, which protects Padstow Harbour. Similar greenstone forms the curious islands known as GuUand Rock, Newland, and the Mouls, and it also appears in Rumps Poiiit and Cliff Castle. The lavas are somewhat softer than the greenstones, but still tend to form headlands with sandy bays between. But when the lavas disappear near Port Isaac we again meet with an even sweep of slaty cliff, only indented here and there by valleys whidh give access to a short piece of foreshore. The coast scenery is illus- trated by numbers 431-438, 448, 521, 522 in the Survey Collection of lEngiish photographs. 90 CHAPTER VIII. ECONOMICS. Except for a few large quarries, some china-clay, works, and three or four mines the economic working of minerals is not an important industry within the area here dealt with ; most of the levels, shafts, and spoil-heaps dotted over the country belong to mines of so little importance that no record has been kept, or if there is a record it shows that the mine produced too little ore to have been worked at a profit. Many of the so-called mines were in reality nothing but ' trials,' often opened for speculative pur- poses and closed after a considerable amount of capital had been sunk. As the stone-industry is that of greatest importance we will take it first. Building Stone. Though so large an area is coloured on the map as granite, the parts sufiiciently sound for engineering purposes are compara- tively small, and the quarries are so far from water-carriage as to make land-transportation a serious item in the cost. One large quarry, that of De Lank, possesses sound granite of so excellent a quality that it can overcome this disadvantage. The working of this quarry is greatly facilitated by the occurrence of a water- fall close by, from which power is obtained for the cutting and dressing of the stone. The stone here worked is appireciably finer in grain than most of the other masses of the Cornish granite, or than the rest of this mass, and it is hard and specially durable. It is composed of interlocking crystals of moderate size, with no marked arrangement in parallel planes ; thus it possesses in every direction a uniform resistence to crushing, which is a very im- portant character in engineering work. The Eddystone, Bishop's Rock, Smalls Rock, and Beachy Head lighthouses have been built of stone from this quarry, and it has also been used for stone- work in the Tower and Blackfriars Bridges over the Thames ; it has been employed in docks in Hull and Bombay, as well as in a great number of public buildings. The granite is pale-grey, and the small size of the white felspars imparts to it an appearance more attractive than that of the coarser rock. One of its most striking features is the almost total absence of patches of colour different from that of the body of the rock, and in this respect the De Lank granite stands almost unrivalled. Polished slabs and dressed blocks from this quarry can be seen in the Museum of Practical Geology, where also have been deposited a series of photographs to show the method of working (numbers 515-520). The other granite-quarries in this area are only worlied to supply the local demand, and the same is the case with the elvans. Over most of the district the building-stone consists of the more massive parts of the Devonian slate, which are either used so that the flat vertical joints are exposed to the weather, giving a curious mottled appearance to the building ; or more rarely the soft slates are dressed with the axe. Sawn stone is seldom employed, except where waste material, with the original sawn faces, is taken from BTJILDING STONE. 91 the tips of tlie slate-quarries. These quarries, such as Delabole, the Camel Slate-quarry, and Pawton quarry, supply in their waste an enormous quantity of ready-quarried building-stone of good quality, and this is being used in Padstow and Wadebridge, besides having supplied material for the bridges, &c., on the railway. For fine tooling and for standing weather no rock in Cornwall is better than that of Cataclews, if carefully selected. It is a dark-grey fine-grained stone, full of spangles of black mica, and is easy to work. Well-preserved ancient carved work, which has been exposed to the weather for several centuries, can be seen in St. Merryn church, and a good deal has been used around Padstow, and it is still employed to a small extent. It somewhat resembles in colour and grain the Polyphant stone, but does not tend to exfoliate as is the case with that rock. Though little used, on account of its hardness, attention should be drawn to the greenstone of Stepper quarry. The stone is much harder than that of Cataclews, but looks as if it would stand the weather equally well, and would take a good polish. It is obtain- able in blocks large enough for the drums of columns, which is a very unusual circumstance in the greenstones and has prevented their use for ornamental work. Squared blocks of the Cataclews and Stepper rocks have been placed in the Museum of Practical Geology ; their petrological characteristics have been described on pp. 42, 45. Slates and Flags. Roofing slates and flag-stones have been quarried on an exten- sive scale at Delabole, at the Camel and Penquean slate-quarries (on the banks of the Camel), near Pawton, and to a small extent in other quarries where the cleavage is fairly regular. As regards the first three quarries, the slate belongs to the Upper Devonian, and may be on the same horizon, though slight metamorphism by the granite intrusion renders the Delabole slate somewhat harder than the others. The Pawton quarry seems to be lower in the series, and is probably Middle Devonian ; but the scarcity of fossils in beds where the slate is of economic value makes exact correlation somewhat difficult. The Camel, Penquean, Pawton, and coast quarries are not now worked, but the Delabole quarries still turn out a large supply of finished slates. For over three centxiries North Cornwall has been famous for its roofing slates, and its fame is still maintained by the high quality of the slates produced at Delabole. Travellers on the North Cornwall Railway, when passing Dela- bole, have their attention attracted by the magnitude of the great excavation, while visitors carry away as mementoes speci- mens of the so-called ' butterflies ' {Spirifer vernewili). These fossils are of much greater interest than the majority of their purchasers are aware, for they fix the geological age of the beds in which they occur. They are Upper Devonian; but on this evidence alone it is impossible further to correlate thepi with rocks of neighbouring areas. Mention, however, has already been made of certain beds of blue-grey slate, which graduate 1S999 G 92 GEOLOGY OF PADSTOW. insensibly into the highly micaceous and metamorphosed phyl- lites of tlie Slaughterbridge type (see p. 32) ; also of the spotted grey phyllites occurring near Meadrose, next to the Delabole type. But when evidence of thermometamorphism is sought for in the Delabole slate, the search is unsuccessful; except in so far as it bears indefinite indications of an influence which has rendered the slate slightly different from the normal unaltered killas of contiguous regions. Workable roofing slate is not confined to one small area; it occupies a wide strip of land running north-westward towards the sea, as is shown by the series of ancient quarries between Delabole and Tintagel. This belt follows the course of the metamorphosed killas described on pp. 28-33; it may, therefore, be taken to have been influenced by the same agency, though to a slighter degree. The excellent quality of the slate was noticed in 1602 by Carew in his ' Survey of Cornwall,' and he further mentions a con- siderable export of the commodity hoth to British and Continental ports. By the middle of the eighteenth century the quarrying operations had opened up an excavation 300 yards long by 100 yards wide.' This pit was only one of the five which were amalgamated in 1837 to form the present quarry, which is now approximately 1,600 yards long by 350 yards wide. The shape of. the quarry and the manner of working it have been decided by the dip of the cleavage of the slate, which is 24° S.W. — the bedding dipping at a higher angle of 30° S.W. ; the workings have thus tended to elongate in the direction of the strike. This strike is rendered obvious by a band of less productive rock some 45 feet thick, which can be traced through the quarry from near llie top at the north-east end downwards to the south-west corner. Above this bed the slate is known as the " old Delabole," and has been worked to a depth of 450 feet, while below it a third bed has yielded slate of even better quality than the top bed. This lower bed has been quarried, chiefly on the north-east side, to a depth of over 100 feet, and a recent trial-bore has proved it to 70 feet below the present bottom of the quarry. Rubble covers these beds, often to a thickness of 20 to 60 feet. Joints cross the slate in several directions, the chief ones running north and south, the smaller ones east and west. The master joints are termed ' shorters,' while the smaller are ' ratchells,' the latter being ' tight ' or ' loose ' ratchells. The qualities which render the slate valuable are its lightness, durability, impermeability, and its permanence of colour. Three beds of different quality occur in the quarry, but each of these differs from place to place. A full account of the method of working the quarry and of the preparation of the slates was published in 1898'; about 500 men are now employed. The Camel Quarry has been closed over '30 years. It was worked to a depth of 360 feet from surface. The best slate was found on the east. Penquean Quarry (close to the Camel Quarry) ' Borlase, ' Natural History of Cornwall,' 1758. ' Quarry, June, 1898. Numbers 505-514 in the Survey Collection of English photographs illustrate this quarry. ROAD MKTAL. 93 was sunk 100 feet. It is said to have paid as a small qiiarry fiuploying five men, Wt did not succeed as ;i company. Both these quarries seem to have produced a slate softer and less fissile than that of Delabole, and more fitted for flags than for roofing-. A great deal of it can be seen in the neighbourhood. EoAD Metal. Road metal for local use is worked in so many places that it would take too much space to give a list of them ; nearly every- where stone of fair quality can be obtained within a short dis- tance. The best stone of all, and the only one which pays to export, is the hard intrusive greenstone. Next come the elvans, which vary considerably, and the harder silicified parts of the vesicular lavas. Then may be classed together the harder parts of the granite, the rocks of the metamorphic aureole, and the grit or quartzite of the Staddon Grit; but all these latter tend to crush under heavy traffic. The greenstones are worked wherever they can be obtained, though one occasionally hears of a stone rejected as too tough and expensive to break. This objection, however, does not apply to the stone used for roads near a large town, where traffic is very heavy, and much road-stone is shipped from the only quarry in the district that has convenient water-carriage. This quarry, at Stepper Point, directly adjoins the deep channel of the Camel; it is in a particularly wide dyke of sound, tough greenstone, with no shearing. The stone is broken by machinery on the spot, and can be shot direct into the vessel ; but, unfortunately, this narrow channel is not a safe place for vessels of any size to lie, and lighters are now commonly used. Much of the broken stone is sent to various parts of South Wales, especially to the large towns in the eastern parts, where there are no hard igneous rocks. One singular source for road-metal may be mentioned, though it is not of much economic importance. At Gravel Caverns, on the east side of Padstow Bay, the sea, especially in winter, drives in considerable quantities of beach material of unusual character, and this is systematically removed. This gravel was found to. consist, by weight, of quartz, 51 per cent., chalk-flint and greensand-chert, 35 per cent., greenstone, 8'3 per cent., and slate, 5'7 per cent., with a very small quantity of shell fragments. Flint-beaches of this peculiar character occur also in Mount's Bay and at Gunwalloe, in the Lizard ; they seem to be derived from some old Tertiary deposit now beneath the sea.' China-clay. As already pointed out (p. 51), kaolinised granite occurs within Bodmin Moor, but in many cases the alteration has not proceeded far enough to produce a marketable china-clay. The change may be taken as complete when the clay, on being rubbed between the fingers, leaves a film that is pure white in colour and has the ' See Reid, hi ' Geology of the Land's End District,' {Mem. Geol. Sm-v.), 1907, pp. 68-71. 15999 ^ 2 94 GEOLOGY OF PADSTOW. unctuous feel of French-chalk, with an entire absence of gritti- ness. The origin of this clay has long been a matter of dispute.^ Briefly put, there are two opposed views : one that the alteration is the result of hot gases ascending from below ; the other that it is decomposition acting downward from the surface, or ' weather- ing.' The latter view would be tenable if the kaolinised granite contained the products of normal decomposition; as a matter of fact it does not do so. Weathered granite is brownish in colour, and examination reveals the persistent presence of decomposed crystals of greenish brown mica. No such mineral is ever asso- ciated with marketable china-clay. The brown colour is due to the decomposition of the brown mica as well as of the original iron ores, both giving rise to brown oxide of iron which stains the material in which it occurs. The prepared china-clay owes its whiteness to the fact that any iron-bearing material in the original rock has been absorbed in the formation of minerals that will not readily decompose, and can be removed by washing. Further, the white clay has been found to continue to any proved depth, with only one important exception. Should an iron- bearing lode cut the clay, the china-clay in its vicinity is iron- stained and in most cases rendered locally useless. The altera- tion must therefore have been effected before the formation of the lode and obviously this can not be of recent formation. The great age of the alteration is perhaps best shown by the dominant influence of the presence of these kaolinised masses in the development of the main features of the scenery. The granite area may be described as a mass of rising ground intersected by a considerable number of valleys, the peculiarity of which is that they are composed of alternations of comparatively narrow and broad portions. In the latter the stream has always a very slow fall, the ground is almost waterlogged and, where undis- turbed, is covered by a thick sheet of peat ; in fact it is a typical Cornish marsh. ^ Wherever the floor or ' shelf ' of these marshes has been tested the granite has invariably been found to be more or less kaolinised ; so soon as this kaolinisation ceases the valley at once narrows, or, as the stream tinners put it, " the sides of the hills come together." As all the larger valleys within the granite show much the same features, one, the Glynn Valley may be described in detail, and this description is applicable to all. Starting then at the watershed, the stream rises in a broad but shallow peat-covered hollow on the north side of the road to the north-west of Temple. The floor or shelf of this shallow hollow is known to be kaolinised, but it would be almost impossible to work, owing to the small fall and the want of water; moreover, the thick peat would cause the little water there is to be dis- coloured and to spoil the clay. Quitting the hollow and ascend- ing the surrounding slopes, the granite to the south-east is singularly massive and free from kaolinisation or even schorl veins. The rock on the other slopes is not no solid, but it is ' See 'Geology of the Country around Bodmin and St. Austell,' {Mem. Geol. Sun.), 1909, Chap. IX. ' Shown in Nos. 494, 495 of the Survey Collection of English photographs. CHINA CLAY. 95 easily seen to be practically free from kaolinisatiou. Tlius the kaolinisation extends practically across the whole shallow depres- sion, but so soon as the kaolinisation ceases the ground at once rises. Following down the stream and crossing the road another broad marsh is reached within which the granite is clearly kaolinised, for it has been worked for china-clay in two large and deep pits. These were closed owing to the heavy cost of transport of coal and of the dried china-clay, but the quality is so good that one has recently been reopened. Here again so soon as the surround- ing slopes, now higher and steeper, are ascended, the kaolinisa- tion practically ceases. In this case, however, it can be proved that the alteration extended along cracks and veins for some dis- tance from the main altered mass, for a few vein-like patches of kaolinised granite are exposed in an elvan quarry close by. Minute threads of kaolin occur also in the elvan, but they are rare and not easily found ; apparently the compact elvan resisted the passage of the gases better than the granite. Further down the stream the kaolinisation again ceases and the valley at once becomes narrow, as the map shows ; but it widens out again and has a broad peat-covered base still further south. Here again the china-clay has been worked. The position of these pits, surrounded by peat and peaty ground, makes it difficult to keep bad water out, a»id numerous small openings have been made at the foot of the valley banks to see if good clay could not be obtained a little above the peat-level ; all were failures ; but they serve to show the last trace of the kaolinisation and how quickly the ground rises where the alteration ceases. It is peculiarly clear in this part of the valley that the marsh coincides with and is absolutely dependent on the kaolinisation of the granite. A little south of the old pit the kaolinisation ceases and once more the valley is narrow and steep-sided and continues so to the margin of the granite. The origin of the marshes or peat-covered slow- falling portions in the valley now becomes clear ; they are all due to the more or less complete kaolinisation of the granite on which they rest ; and this obviously took place anterior to the formation of the valleys. Indeed a study of the map points to the conclusion that the entire valleys themselves are dependent on this alteration and are formed by the linking together and deepening of groups of the original shallow depressions due to the kaolinisation. The idea of this peculiar type of alteration being due to modern agencies is thus clearly tmtenable ; it is older than the develop- ment of the scenery within the granite. Disti'Ibvtioii of the Chin(i-cJ VIII, 1836, p. 253. 104 GEOLOGY OF PADSTOW. granite. The whole width of the tin-bearing belt was dug out when the price of tin was high. Eventually the rock become harder, and the level of the adjacent stream being reached rendered pumping necessary. Within recent years a shaft has been sunk and the workings carried down to a depth of over 50 fathoms. As the lode was followed underground away from the granite into the killas the amount of tin rapidly decreased and the vein became practically a copper lode. The date of the first open workings on this lode is unknown. It was followed on the hill above Warleggon Down, and here also was sporadically worked as an open mine, where the vein became rich ; some of these workings are still open, and one of the largest is noted on the six-inch map as a ' quarry.' In the adjacent valley to the east of Warleggon a level was driven along the lode, starting in the wood and quite at the foot of the hill. A considerable amount of ore is said to have been obtained. The vein-stone is a ' peach ' of the black or quartz-schorl type. The amount of tin and copper taken out, as far as statistics are available, is given in the table below. Treveglos Mine. — A north and south lead-lode occupying part of the fault which passes southward through Trevorgus. It has been worked for about half a mile close to St. Merryn, but does not appear to have been tried further south. Trevillick Mine. — On a N. and 8. lead lode. Trevinnick Mine. — An E. and W. lode, mainly in lava, worked for antimony and lead. The jamesonite occurs in scattered nodules in gossan. A large specimen obtained at the mine and placed in the Museum of Practical Geology was assayed by Dr. Pollard and proved to contain 2 oz. 16 dwt. of silver per long ton. Probably the antimony ores from other mines will yield silver, but it does not seem to have been recorded. Some pale-green, almost colourless crystals obtained at the same time were examined by Mr. H. H. Thomas. They proved to be pyromorphite (lead chlorophosphate, 3 Pba PaOg. Pb OU). Trewane Mine. — 3 shafts on a N, and S. lode. Teewetha. — A short NNE. lode yielding antimony has lately been reopenedj Wheal Whisper, see Treveddoe Mine. Whiteweli, Mine. — On a NNE. lode, apparently a continuation of Treburgett. Silver-lead. Output op Tin and Copper. Name of Mine. Black Tin. Copper Ore. Copper. Period of Working. Tons. Tons. Tons. Ambrose Lake li 150 8 1872 to 1875. Bray, Wheal 508 15 In 1857 and 1858. Goonzion 18 8 i In 1863 and 1864. Great Onslow Consols... 935 31 Between 1853 and 1856, and in 1863. Hammett 2 In 1870. Hobbs Hill Mine 24 Between 1872 and 1874. Mary, Wheal 11,910 700 1828 to 1851. Molesworth, Wheal ... 10 1 T In 1886 and 1887. Robins, Wheal 27 Between 1852 and 1855. Tregeagle 103 Batween 1870 and 1874 ; 1884, 1891. Treveddoe (formerly 847 65 6 Copper in 1823, 1824, Wheal Whisper.) 1862, 1904 and 1906 ; tin between 1859 and 187.3, 1885 and 1887, 1899 and 1907. MINING. 105 Output or Minerals other than Tin and Copper. Name o£ Mine. Lead Ore. Lead. Silver. Iron Pyrites. Spathic Iron and Brown Anti- mony Ore. ■ Other Minerals. Period of Working. Haematite. Tons. Tons. Ozs. Tons. Tons. Tons. Ambrose Lake 80 64 320 36 tons crude and refined arsenic. 1875, 1876. Bodannon 4 In 1884. Valued at £40. Boys, Wheal . . 95 Between 1774 and 1776. In 1845. Camellord 180 108 Carnevivian .. 20 12 140 In 1862. Carnewas 940 In 1871 and 1874. (Brown Haema- tite.) Credis and Little 37 In 1869. Petherwiok Great Onslow Consols 4,950 Between 1856 and 1863. North Porthilly .. 28 21 In 1862 and 1863. Fadstow Consols . . 75 56 270 in 1876 and 1877. Pawton Mine 49,350 5,372 Between 1861 and 1864. (Brown Haematite.) In 1873 and 1874. Pawton and Tremoor 9,626 In 1866. Pengenna 16 In 1861 and 1862. Pentire Glaze 955 6.32 19,085 Between 1850 and 1857. The silver between 1853 and 1857. Polzeath 6 4 In 1855 and 1856. Treburgett , . 2,180 1,640 9,530 120 02 44 tons ol zinc ore. Between 1871 and 1881. rreVinnick . . 30 21 6 In 1876, 1892 and 1893. Antimony ore contained 43% antimony. In 1853, 1855 and Tregardook . . 60 30 690 1860. Tregorden 53 33 Between 1849 and 1851. Mine. Ambrose Lake Mine Bodannon Boscarne B07B, Wheal Bray, Wheal Cabilla Carnevivian Mine Came was CredisMine Dinham Mine ... Goonzion Greaj! Onslow Consals Great Tie>zeddoe Guddrabri'd'ge Mine Guttbridge Hardhead Mine... Hammett Herbert, Wheal... Hobbs Hill Mine Hurtstooks Mine Legossick Mine... Little Petherick Mine Index of Parish. St. Neot. Bndellion. Bodmin. Endellion. Altarnun. Warleggon. St. Eval. Padstow. St. Minver. St. Neot. St.Breward. Warleggon. Padstow. St. Minver. Warleggon. St. Neot. Cardinham. St. fssey. Padstow. the Mines. Mine. Mary, Wheal ... North Porthilly... Old Treburgett... Old Trewetha ... Onslow Consols... Padstow Consols Pawton Mine ... Pendoggett Pengenna Pentire Pentire Glaze ... Poltreworgey ... Polzeath Bobins, Wheal ... Rose Tresungers Ronghtor Mine . . . Sisters, Wheal ... Trebetherick Treburgett Trefresa Tregardock Tregeagle Tregelles Treglyn Mine ... Tregorden Treknow Parish. St. Neot. St. Minver. St. Teath. Endellion. St. Breward. Padstow. St. Breock. St. Kew. St. Teath. St. Minver. ») Endellion. St. Minver. St. Neot. Endellion. St. Clether. St. Neot. St. Minver. St. Teath. St. Minver. St. Teath. St. Neot. St. Kew. St. Minver. Egloshayle. Tintagel. Missing Page MINING. 107 Stream-tin and Wolfram. In addition to the underground mining just described there as formerly, as already observed, an important industry in the ashing of the alluvial deposits for stream-tin. This " grain- n " was of the best quality and always fetched the highest rices, for out of it had been dissolved all soluble impurities, such I arsenic, copper, lead, and zinc, which might injure the metallic a prepared by the processes in use during the Middle Ages. As ng as the stream-tin was obtainable cheaply and in large lantity there was little temptation to work the lodes, which, ccept at their weathered outcrops, must have been difficult to ine with the tools then in use. Thus it is that the early records )ntain no mention of copper, which must have been found soon 'ter the tin-lodes were attacked, for a large proportion of these des contain some copper, especially where this ore is concen- ated, near the surface. This mixed ore may have been the lurce from which bronze was made during the Bronze Age ; but, so, the early mining died out by Caesar's day, and the Roman >vival in the third century appears to coincide with the Roman lanufacture of pewter, not of bronze, and the ore obtained was ream-tin. The origin and mode of occurrence of these alluvial tin-bearing jposits has been described in Chapter VII. Most of these on odmin Moor have already been worked, though certain extensive aterlogged fiats are only now being exploited by the modern ethod of dredging. The lower reaches of the Camel, however, lem never to have been worked, and must contain a large quan- ty of tin beneath the sea-level. It will be worth while to take lis into account if any proposal is made to improve the navigation y dredging. Even Padstow Harbour, if cleared out, might be lund to yield much tin from the bottom of the alluvium. It is 3t easy to make out how much of the alluvium above Wade- ridge has been tinned ; there are no early records, and so much aste has come down from higher workings that the field-evidence IB been obliterated. Great part of the accumulation of sand- inks and the destruction of the harbour between Wadebridge id Padstow is probably due to this washing. Wolfram occurs in considerable quantity, but till recently there as no demand for it, and unless in large lumps it was almost ipossible to separate from the stream-tin before the invention of le magnetic separator. Thus not only have the wolfram deposits sen neglected, but any tin-lode or stream-tin containing more lan a small percentage of wolfram was rejected. Lately there IS been a great demand for wolfram, for the manufacture of ingsten-steel, and some of the alluvial deposits at the north end ■ Bodmin Moor have yielded considerable quantities. Commencing with the north end of the granite, it has been met ith in considerable quantity in the stream deposits at the north d of Stannon Marsh.* It is again locally abundant in the attern Hill area, further east. For a considerable distance from ' See Barrow, ' The High-level Platforms of Bodmin Moor and their relation the deposits of Stream-tin and Wolfram.' Quart. Jown. Geoi. ggp., 1. LXIV. (1908), pp. 384-400. 16999 a 108 GEOLOGY OF PADSTOW. these points the ground has not been tested for this mineral, but it proved to be present in the long marsh to the north-west oi Garrow Downs. The Beacon Hill, on the west edge of the map is penetrated by one or more of the wolfram-bearing -veins that have been worked to the east, and one of these veins is said to have been recognised on the left bank of the Fowey, due east of Codda Farm ; and wolfram is certain to be present in the alluvial deposits here. To the south of this for a great distance the ground has not been proved ; but wolfram is practically absent over the area lying south of a line drawn east and west through Temple. A great number of veins have been tested from time to time in the southern part of the granite, and these show that wolfram is rare or absent in this area. The alluvium has been tested at CoUiford Downs, north of St. Neot, and a small proportion of wolfram only is here associated with the tin ore; enough occurs to suggest its greater abundance further north. Nothing is as yet known as to its occurrence over the belt of ground that lies on the two sides of the high road in the Temple area. Water Sltpply. In the granite of Bodmin Moor this region contains an ideal source of supply, for the water is very pure and soft, though if collected on the surface, slightly peaty. Another fair gathering ground is the high ridge of the Staddon Grit, between St. Breoek Downs and the coast. The centres of population, how- ever, lie mainly at some distance from these sources, and most of the towns and villages are supplied from local wells in the slates. These slates are so impervious that most. of the rain falling on them runs ofE at once, and comparatively little is stored up and available for the summer. Thus, notwithstanding the. considerable rainfall, two or three weeks of drought is sufficient to cause a good deal of difficulty. The growing watering-places in particular find it increasingly difficult to supply the summer demand, which comes just at the time when the slates contain least water. The water in the slate, in addition to its small quantity, is usually hard, with dissolved carbonates or a good deal of iron, and if .the gathering ground is at all near the sea, it contains also a perceptible quantity of common salt. Bodmin is supplied from Bodmin Moor, and the question of supplying Camelford and Delabole from a similar source is under consideration. But there is still a large area of granite moor available. The old tin-mines on this moor form excellent col- lecting-galleries and reservoirs for water-supply purposes, as most of the tin-lodes are free from the soluble metals so objection- able in drinking-water, and contain no carbonates. The water from the granite is exceptionally pure. A reference to the ' notes on the mines ' (pp. 100 to 104) will show what old mines are available ; though of most of them no plans or pumping-records have been kept, and we have no means of estimating the amount of water each would yield. The flow from the adit will often give some idea ; but it must be remembered that a considerably larger supply would often be obtainable below adit-level. 109 BIBLIOGRAPHY. 1602. Carew, R.— Survey of Cornwall. 4to. (Reprinted in 1769). 1681. Grew, De. N.— Catalogue and Description of the Natural and Artificial Rarities belonging to the Royal Society and preserved at Gresham College, fol. London. 1758. BOELASE, Rev. W.— The Natural History of Cornwall, fol. London, p. 129. 1797. Anon. Specimens of British Minerals selected from the Cabinet of Philip Rashleigsh. 4to., London, (p. 34.) 1804. BouENON, Count de.— Description of a triple Sulphuret, of Lead, Antimony, and Copper, from Cornwall. Phil. Trans., vol. xciv, p. 30. 1808. Smithson, J.— On the Composition of the Compound Sulphuret from Huel Boys, and an Account of its Crystals. Phil. Trans., vol. xcviii, p. 55. 1817. CONYBEAEE, Rev. J. J.— Notice of Fossil Shells in the Slate of Tintagel. Trans. Geol. Soc. Lond., vol. iv, pp. 424-426. 1818. Davy, Sie H.— ' On the Geology of Cornwall.' Trans. Roy. Geol. Soe. Com., vol. i, p. 46. 1823. CoNYBEAEE, Rev. J. J.— On the Geology of Devon and Cornwall. Ann. of Phil , Ser. 2, vol. v, p. 184, and vol. vi, p. 35. 1825. Caene, J. — Deposition of Sand on the Northern Coast of Cornwall. Trans. Roy. Geol. Soc. Corn., vol. iii, 136-149. 1832. BOASE, De. H. S.— On the Geology of the Eastern District of Cornwall. Trans. Roy. Geol. Soc. Corn., vol. iv, p. 174. 1839. De la Beche, Sie H. T.— Report on the Geology of Cornwall, Devon and West Somerset. Geol. Surv., 8vo., London. Williams, Rev. D. — On as much of the Transition or Grauwacke System as is exposed in the counties of Somerset, Devon and Cornwall. Proo. Geol. Soc, vol. iii, p. 115. 1840. On as much of the great Grauwacke System as is comprised in the group of West Somerset, D*von and Cornwall. Ibid., vol. iii, p. 158. On the Geology of Devon and Cornwall. Phil. Mag., Ser. 3, vol. xvi, p. 59. 1841. Pattison, S. R. — Slate from Tintagel containing organic remains. Rep. Roy. Inst. Corn., pp. 26-27. Phillips,- Peop. J.— Palseozoic Fossils of Cornwall, Devon and West Somerset. Geol. Surv., 8vo., London. 1842. Williams, Rev. D.— On the Stratified and Unstratified Volcanic Products in the neighbourhood of Plymouth. Rep. Brit. Assoc, for 1841, pp. 61, 62. 1843. Supplementary Notes on the true position in the " Devonian System " of the Cornish Killas. Phil. Mag., vol. xxi, p. 25. On the Stratified and Unstratified Volcanic Products of the West of England. Trans. Roy. Geol. Soc. Com., vol. vi, p. 173. 1844. On the Killas group of Cornwall and South Devon. Phil. Mag Ser. 3, vol. xxiv, p. 322. 1846. On the Killas Group of Cornwall and South Devon. Trans. Roy. Geol. Soc. Corn., vol. vi, p. 122. 1847. Pattison, S. R.— On the Geology of the Tintagel District. 3ith Ann. Rep. Roy. Geol. Soc. Com., pp. 3-12. Shaepe, D. — On Slaty Cleavage. Quart. Journ. Geol. Soc, vol. iii, p. 74. 1848. Pattison, S. R. — A brief description of the coast of Cornwall, between the Padstow River and Perran Sands. Trans. Roy. Geol. Soc. Com., vol. vii, p. 47. 1849. Whitley, N. — On the Remains of Ancient Volcanoes on the North Coast of Cornwall in the parish of St. Minver. 30ih Ann. Rep. Roy. Inst. Corn., p. 60. 1852. Sedowick, Rev. Prof. A. — On the Slate Rocks of Devon and Cornwall. Quart. Journ. Geol. Soc, vol. viii, p. 1. 1854. Pattison, S. R. — On Auriferous Quartz-rock in North Cornwall. Quart. Journ. Geol. Soc, vol. x, p. 247. 1859. Henwood, W. J. — Notice of the Submarine Forest near Padstow. iOth Ann. Rep, Roy. Inst. Corn., pp. 17, 31. 16999 H 2 110 GEOLOGY OF PADSTOW. 1865. Pattison, S. R— Geology of the Tintagel District. Trans. Roy. Geol. Soc.Coj-n., vol. ,vii, p. 3. j -ci t 1868. HoLL, Dii. H. B.— On the Older Books of South Devon and Jiast Cornwall. Qtmrt. Journ. Geol. Soc, vol xxiv, pp. 400-454. , 1871. Phillips, J. A.— Microscopic constituents of certain Cornish RocJss. Land. Edin. and Dubl. Phil. Mag., 4th Ser., No. 27, pp. 95, 96. 1875. The Books of the Mining Districts of Cornwall, and their Eelation to Metalliferous Deposits. Quart. Journ. Geol. Soc, vol. xxzi, p. 319. 1876. Collins, J. H.— On the Pawton Iron Mine. Heps. Min. Assoc. Corn, and Devon, p. 26. 1878. Phillips, J. A.— On the so-called " Greenstones " of Central and Eastern Cornwall. Quart. Journ. Geol. Soc, vol. xxxiv, p. 471. 1880. USSHER, W. A. E.— On the Beoent Geology of the Cornish Coast near Padstow. Trans. Roy. Geol. Soc Corn., vol. x, p. 58. 1886. EuTLEY, P. — On some Eruptive Rocks from the Neighbourhood of St. Minver, Cornwall. Quart. Journ. Geol. Soc, vol. xlii, p. 392. 1888. Teall, Dr. J. J. H. — British Petrography. 8vo., London. 1889. HnxcHiNGS, W. M. — Notes on Altered Igneous Rocks of Tintagel, North Cornwall. Geol. Mag., pp. 53-59, 101-109. On the occurrence of Ottrelite in the Phyllites of North Cornwall. Geol. Mag., p. 214. 1894. Fox, H. — Notes on the cherts and associated rooks of Bound Hole Point (Permizzen Point), Cataclews Point and Dinas Head, West of Padstow. Trans. Roy. Geol. Soc Corn., vol. xi, p. 687. 1895. On some fossils from the Coast Sections in the Parishes of Padstow and St. Merryn. Trans. Roy. Geol. Soc Corn., vol. xi, p. 634. On some Nodular Concretions resembling Fossil Wood and Fossil Fish, Launoeston. Trans. Roy. Geol. Soc Corn., vol. xi, p. 684. On a Soda Felspar Bock at Dinas Head, North Coast of Cornwall. Geol. Mag., p. 13. — On some Fossils from the coast sections in the Parishes of Padstow and St. Merryn. Trans. Roy. Geol. Soc Com., vol. xi, p. 634. 1896. On a Soda Felspar Bock at Dinas Head, near Padstow. Journ. Roy. Instit. Corn., p. 183. 1899. — — Supplementary Notes on the Cornish Badiolarian Cherts and Devonian Fossils. Trans. Roy. Geol. Soc Corn., vol. xii, pp. 278-282. Geological Notes. Trans. Roy. Geol. Soc Corn., vol. xii, pp. 347-354. 1900. Bowman, H. L. — On Mouazite and associated minerals from Tintagel, Cornwall. Jour. Min. Soc, vol. xii, p. 358. Fox, H., and G. J. Hinde. — Notes on the G«ology and Fossils of some Devonian Bocks on the North Coast of Cornwall. Geol. Mag., pp. 145-152, and Trans. Roy. Geol. Soc Com., vol. xii, pp. 342-361. 1901. Fox, H. — ' On the Distribution of Fossils on the North Coast of Cornwall south of the Camel.' Trans. Roy. Geol. Soc. Com., vol. xii, pp. 535-644. 1903. ^— Some Coast-sections in the Parish of St. Minver. [With Notes by J. Parkinson on the Igneous Bocks.] Trans. Roy. Geol. Soc. Corn., vol. xii, pp. 649-682. Parkinson, J. — The Geology of the Tintagel and Davidstow District. Quart. Journ. Geol. Soc, vol. lix, pp. 408-428, figs., pi. xxv (Geol. Map). Woodward, H. — On Two Trilobites from the Devonian Slates of Cornwall. Geol. Mag., pp. 28-31. 1904. Fox, H. — Geological Notes. No. 2. Supplementary Notes on the Distri- bution of Fossils and Bocks on the North Coast of Cornwall. Trans. Roy. Geol. Soc. Corn., vol. xii, pp. 753-759. ^— Supplementary Notes on some Coast-sections in the Parish of St. Minver. Trans. Roy. Geol. Soc. Corn., vol. xii, pp. 747-752. Flett, J. S. — ^Notes on the Igneous Bocks in St. Minver Parish. Trans. Roy. Geol. Soc Corn., vol. xiii, pp. 29-33. 1905. Bather, Dr. F. A. — Sympterura Minveri, n.g. et sp., a Devonian Ophiurid from Cornwall. Geol. Mag., pp. 161-169, BIBLIOGRAPHY. Ill 1905. Crick, G. C— On some Fossil Cephalopoda from North Cornwall, collected by Mr. Howard Fox. Geol. Mag., pp. 154-160. Fox, H. — Devonian Fossils from the Parish of St. Minver, North Cornwall. Geol. Mag., pp. 145-150. Further Notes on the Devonian Rocks and Fossils in the Parish of St. Minver. Tram. Roy. Geol. Soc. Corn., vol. xiii, pp. 33-35. Woodward, H.— Notes on a Series of Trilobites obtained by Mr. Howard Fox from the Devonian of Cant Hill, St. Minver (Cornwall). Geol. Mag., pp. 151-154. RUDLBR, F. W.— A Handbook of British Minerals. [Especially pp. 59 ; 62-64.] USSHER, W. A. E. — Notes on the Upper Devonian Rocks in St. Minver Parish. Trans. Roy. Geol. Soc. Corn., vol. xiii, pp. 85-87. 1906. Fox, H.— The Variegated Slates of North Cornwall. Trans. Roy. Geol. Soc. Corn., vol. xiii, pp. 127-134. t907. UsSHER, W. A. E.— Geology of the Country around Plymouth and Liskeard. Mem. Geol. Surv. 1908. Barkow, G. — The High-Level Platforms of Bodmin Moor and their Relation to the Deposits of Stream-Tin and Wolfram. Quart. Journ. Geol. Soc, vol. Ixiv, pp. 384-400. Barrow, G., and H. H. Thomas. On the occurrence of metamorphic minerals in calcareous rocks in the Bodmin and Camelford areas, Cornwall. Min. Mag., vol. xv, pp. 113-123. Lewis, Dr. G. R. — The Stannaries (Harvard Economic Studies, Vol. TIL), 8vo., London. Reid, C, and H. Dewey. — The Origin of the Pillow-Lava near Port Isaac in Cornwall. Quart. Journ. Geol. Soc, vol. Ixiv, pp. 264-272. 1909. Dewey, H. — Oil Overthrusts at Tintagel (North Cornwall). Quart. Journ. Geol. Soc, vol. Ixv, pp. 265-280. Thomas, I. — A New Devonian Trilobite and Lamellibranch from Cornwall. Geol. Mag., pp. 97-102. The Trilobite Fauna of Devon and Cornwall. Geol. Mag., pp. 193-204. Washington, H. S. — The Submarine Eruptions of 1831 and 1891 near Pantelleria. Amer. Journ. Science, vol. xxvii, pp. 131-150. Photographs. In addition to the photographs reproduced in this Memoir, there are many in the Survey Collection which illustrate this area. The following are selected from the published catalogue as being the most generally useful : — Devonian rooks, 450, 451, 452, 453, 454, 455, 471, 522. Tuff, 455. Pillow-lava, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 523 524, 833, 8a4. Greenstone sills, dykes, and contact altered rocks, 454, 467, 468, 469 470, 471 522 525 526 5'^7 Granite tors, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485 488. Elvans, 472, 473. Tertiary plateaus, 488, 489, 490, 491, 492, 493, 494. High-level marshes, 494, 495. Raised beach, 474, 475. Head, 447, 475, 496, 497. Stream-tin and wolfram workings, 498, 499, 500, 501, 502, 503, 504. 112 Valley erosion, 442, 446, 447, 448, 486, 488, 489, 490, 859, 860. Sand dunes, 449. Coast scenery, headlands and islets, 431, 432, 433, 434, 435, 436, 437, 438, 448, 521, 522. Caves, 528. Padstow Harbour, 430, 439, 440. Delabole slate-quarry, 505, 506. Slate-dressing at Delabole, 507, 508, 509, 510, 511, 512, 513, 514. De Lank granite-quarry, 515, 516. G-ranite-dressing at De Lank, 517, 518, 519, 520. U3 INDEX. Abbott's Hendra, 31. Adinole, 19, 25, 26, 48, 49. 69, 73, 74, 77, 106. Agriculture, 2. Albite, 19, 31, 42-44. diabase, 42, Plate IV. AUanite, 42. Allen River, 43. Alluvium, 1, 2, 83-87. Altarnun, 105, 106. Altered Rocks, 2-5, 12, 15, 17, 19, 28-34, 38, 41, 43, 47-50, 60, 61, 63-77, Plates II., IV. Amble River, 23. Ambrose Lake Mine, 100, 101, 104, 105. Amethystine quartz, 67. Analyses of diabase, 43 ; camptonite, 46 ; essexite, 46 ; iron-ores, 101 ; minettes, 62 ; pillow-lavas, 41 ; pro- terobases, 46 ; slates, 33. Andalusite, 30, 32, 54, 65-69, 74, 75. Anticlines, 2, 3, 32. Antimony, 1, 99, 100, 102-105, 109. Aplite, 60. Arctic climate, 80-83, 86, 87. Area, 1. Arsenic, 100, 105. Aureole of metamorphism, 2, 3, 63-77. Axinite, 72. Ball Hill, 22, 37, 39. Barkevicite, 44. Barras Nose Beds, 28-31, 33, 34. Barratt, W., 4, 6, 8, 10, 20, 21. Bather, Dr. P. A., 4, 9, 21, 110. Beachy Head Lighthouse, 90. Beacon Hill, 108. Bedruthan, 8-10, 12. Biotite, analyses of, 54. Bishop's Rock Lighthouse, 90. Wood, 13. Blackfriars Bridge, 90. Blende, 99-102, 105. Blisland, 47, 56-60, 63, 67, 69, 73, 75, 76, 96, 97. Blount, B. 33. Blown sand, 1, 82, 88, 89. Boase, Dr. H. S., 3, 35, 109. Bodannon Mine, 100, 105. Bodellick, 36. Bodieve, 23. Bodmin, 7, 79, 106, 108. Bodmin Moor, 1, 2, 17. 51-61, 63-81, 85-88, 93-108, 111. Bodrigan Wood, 60. Bohemia, essixites of, 46. Bokelly, 35, 60. Bolventor, 52, 55-57, 78, 87, 88. Bombay Docks, 90. Booby's Bay, 18, 20, 23. Borlase, Rev. W., 92, 109. Boscame, 7 ; mine, 100, 105. Boacastle, 1, 5, 79. Bossiney Bay, 29-32. Bounds Cliff, 27, 79. Bournon, Count de, 109 Bowman, H. L., 110. Boys, Wheal, 103, 105. Bozion, 13. Bray Cottage, 25. Down, 52, 63. Hill, 43, 45, 60. Wheal, 100, 104, 105. Brilon, 26. Bronze Age, 107. Browngelly Downs, 55, 78. Brown Willy, 1,52. Buchiola retrostriata, 18, 20, 25. Budesheim fauna, 20. Building-stone, 15, 60, 90, 91. Burlorne Eglos, 13. Burniere, 22, 23, 25, 39. Bury Castle, 68, 74, 75. ' Butterflies,' fossil, 91. Butter Hole, 22. Buttern Hill, 107. Cabilla, 68, 70. Mine, 105. Calo-flinta, 5, 12, 19, 28, 29, 65, 69-72. Calcite in lava, 38-42. Camel, River, 1, 5, 6, 13-15, 63, 79-81, 83, 84, 87-89, 107 ; cliffs, 22, 37. 43, 61, 63, 68, 69, 75, 93 ; Quarry, 23, 34 91 92. Camelford, 1, 19, 28-30, 32, 37, 38, 51, 53, 58-60, 63, 65, 68-70, 72, 73, 75, 76, 78, 79, 84, 103, 108. Camelford Mine, 105, 106. Camptonite, 45, 46. Cant Cove, 24. Hill, 13, 22, 24, 37, 39, 40, 62, 81. Carbilly Tor, 53, 56. Carboniferous deposits, 2, 3, 5, 29, 33, 34 ; dykes, 43. Cardinham, 15, 53, 56, 59, 60, 67, 74, 75, 84, 96, 106. Carew, R., 93, 109. Carn Bosavern, 54. Carne, J., 109. Carnevivian Mine, 100, 105. Carnewas Mine, 100, 105.' Cams, 51. Carnweather Point, 26. Cartuther, 46. Carwen, 96, 97. Casparpool, 80. Cassacarn, 67, 69. 114 iNDfiX. Cassock Hill, 25, 43, 48. Cataolews, 20, 21, 43,- 46, 89, 91. Caves, 111. Chert in lava, 38, 39 ; radiolarian, 18, 19, 48 ; shelly, 23, 24. Chiastolite, 49, 50. Chiloceras-horizon, 26. China-clay, 2, 59, 86, 93-98. Chloritoid, 30, 32, 33, 65, 66. Church-hill, 39. Cleavage, 2, 3, 10-13, 23, 30, 32, 92, 109. Clerkenwater, 14, 80. Cliff Castle, 38, 39, 42, 89. Climate, 2 ; fortaer arctic, 80-83, 86, 87. Clymenia, 17, 34, 39. Coast scenery, 89, 111. Codda Farm, 108. Coldquite, 96. CoUiford Downs, 87, 108. Collins, J. H., 54, 97, 101, 102, 110. Colour of slates, 15, 17, 21, 23, 25, 27, 40. Colquite Woods, 14. Cone-in-cone, 18. Constantine Bay, 81, 89. Conularia, 11, 17, 20, 24. Conybeare, Rev. J. J., 109. Copper, 1, 99-104. Cordierite, 54, 66-69. Cornubianite, 68. Credis Mill, 100, 105; Crick, G. C, 4, 9, 18, HI. Croan House, 42, 48. Cromlechs, 6. Crookmoyle Rock, 27. Crowdy Marsh, 65, 78, 80. Crugmeer, 22. Culm Measures, 5, 29, 33, 34. Davidstow, 31, 36, 41 ; Moor, 29, 78. Davy, Sir H., 3, 109. Daymer Bay, 21, 25, 27, 28, 34, 82. De la Beche, Sir H. T., 3, 35, 88, 109. Delabole, 79, 108 ; Slates, 28-30, 32-35, 91, 92, 111. Point, 27. De Lank Quarry, 52-65, 68, 90, 111 ; River, 58, 59, 63, 73, 76, 77, 80, 84. Dennis, 13, 22, 37, 39. Point, 29. Depolarising ribbon, 64, 65, 67. Devil's Jump, 51, 63, 84, 102. Devonian, introduction of the term, 3 ; Lower, 2, 5-8, 13, 36 ; Middle, 2, 5, 7-16, 22, 23, 36, 65-68, 91 ; Upper, 1, '2, 5, 10, 11, 13, 17-50, 91. DevonpoTt, 41, 96. Diabase, 2, 19,22, 42, 43, 48, 49. Diggory's Island, 10. Dinas Head, 18-21,48,49. Dinham Creek, 23, 24, 49 ; Mine, 100, 105. Dinnabroad, 27, 34, 40. Dip, 2, 5,-31, 32, 92. Dolerite, 45. Doom Bar, 25, 88. Downhedge Cove, 26. Dozmary Pool, 55, 57, 88. Drainage, 1. Driers, 98. Dunbar, 88. Dunes, 88, 89, 111. Dunmere, 7, 13, 14. Dnnveth, 36. Durfold, 96, 97. Dykes, Elvan, 2, 3, 58-61, 93, HI ; Greenstone, 42, 43 ; Minette, 61, 62. Economics, 90-108. Eddystone lighthouse, 90. Egloshayle, 42, 43, 48, 106. Blvans, 2, 3, 58-61, 93, 111. Endellion, 28, 106. Entomis, 25. Eocene, 93. Epidiorite, 31, 47, 48. Erratics, 6, 7, 81-83, 93. Essexites, 45, 46. Exeter traps, 62. Faults, 17, 21, 22, 27, 51, 102-104. Fawton, 70. Fenterlarick, 60. Flagstones, 11, 91-93. Flett, Dr. J. S., 36, 40, 44-46, 59, 63-77, 110. Flint-gravel, 93 ; implements, 88. Fluorspar, 59. Foliation, 48, 64, 66. Forfarshire, 55. Formations represented, 2. Fossils in adinole, 26 ; in tuff, 39 ; state of preservation of, 10, 12. Fowey River, 1, 83, 84, 87, 108. Fox, H., 4, 8-10, 17, 18, 20, 22, 24, 33, 35, 110, 111. Galena, 99-104. Gam Bridge, 63, 70. Gannel, 62. Garmoe, 65. Garnet, 30, 32, 48, 67, 70, 71, Plate IV. , Garrow Downs, 108. (lazeland, 97. Gentle Jane, 24, 61. Glynn Yalley, 94. Gold, 109. Goonzion Downs, 70, 100 : Mine, 100, .105. Grain-tin, 107. Granite, 2, 51-57, 90, 111 ; quarries, 2, HI ; underground extension of, 3, 27 ; veins, 3, 56, 69. Grauophyre, 61. Graphite, 72. INDEX. 115 Grauwacke, 3. Gravel Caverns, 25, 93. Great Onslow Consols, 100, 104, 105. Greenaway ClifE, 25. Greenstone, 2, 19, 35-50, 75,76, 77, 89, 93, 110 ; altisration by, 73, 74 ; dykes, 22, 43, 111, Plate II. : sheared, 72. Greisen, 57, 60, 61, 75. Grew's Catalogue, 99, 109. Grogley, 7, 8, 13. Grove Landrake, 46. Guddrabridge Mine, 21, 100, 105. Gulland, 42, 89. Gfimbel, C. W., 46. Gunvillibk, 31. Gunwalloe, 93. Guttbridge Mine, 100, 105. Haematite, 31, 41, 100, 101. Hall Farm, 103. HallwortUy, 29. Halwell, 31. Halwyn, 61. Hammett Mine, 100, 101, 104, 105. Harbour Cove, 81. Hardhead Mine, 100^ 105. Harlyn Bay, 20, 89. Haughton, Prof., 54. Haughtonite, 55. Hawker'sCove, 22, 81. Hawkstor-pits, 96. Haycrook, 6, 7, 12. Hayle Bay, 25. Head, 2, 82-87, 111. Heddle, M, F., 55> Heights, 1. Holland, 60, 80. Hellandbridge, 14. Helland Wood, 15. Helsbury, 72, 76. Hendawie, 47. Hendra, 31. Hendraburnick, 30. Hennon Farm, 72. Henwood, W. J., 109. Herbert, "Wheal, 100, 103, 105. Higher Fawton, 103. BasD, 47. Trevorgus, 17, 36. Highlands, 54, 55. Hinde, Dr. G. J., 4, 9, 18, 110. Hobb's Hill, 61 ; Mine, 100, 101, 104, 105. Hogg, T., 103. Holl, Dr. H. B., 35, 110. Holton Farm, 15. HomalonotuSf 6, 10, 20. Hornfels, 66, 69, 72-74. Hornstones, 74. Huel, see name of mine. Hull Docks, 90. Hurstooks Mine, 101, 105, 106. Hustyn Woods, 13. Hutohings, W. M., 4, 28, 33, 35, 110. Ice-action, 81-83, 86. Idocrase, 70, 71. Implements, flint, 88. Inny Eiver, 30, 31^ Iron-ores, 1, 99-103, 105. Iron-pyrites, 100, 105. Jacket's Point, 27. Kaolinised elvan, 59, 85, 86, 95 ; granite, 51-53, 93-97, 99. Kayser, Prof. E , 26. Kellan Head, 39. Kestle Quarry, 42, 48. Keybridge, 63, 76, 77. Eillas, meaning of term, 3. Kitt Hill, .42. Lambrenny, 47. Lamphill Wood, 70, 73. Lamprophyres, 45. Land's End, 80. Lane End, 65. Lanlavery Book, 51, 96, 102. Lanow, 40, Plate IV. Lanteglos, 28, 63, 65. Lava, 21, 23, 27, 28, 30, 31, 35, 93, HI. Lead, 17, 99-104. Legossick Mine, 101, 105, 106. Lemail, 43, 45, 61, 62. Leptynolite, 72, 73. Lewis, Dr. G. E., 99, 111. Limestone, 2, 3, 5, 8. 10, 12, 15, 18-24, 28, 30, 31, 48. Liskeard, 97. Literature, 3, 4. Lithia, 53, 54. Little Cove, 20. Pennatillies, 6. Petherick, 6, 81 ; Mine, 105, 106. Lizard, 93. Lodes, 99-104. Lower Butter Cove, 10. Polpry, 73. Trevarra, 42. Lundy Beach, 48. Luxullianite, 61. MaoAlister, D. A., 100-106. McCoy, F., 9. Magnetite, 31, 41. Mandelstein, 3. Manganese, 100 ; in pillow-lava, 39. Maps, early, 3 ; list of six-inch, iii. Marble ClifE, 5, 18, 19, 21, 22. Marshes, 94-96, 107, 108, 111. Mary,. Wheal, 100, 101, 104-106. Mawgan Forth, 1, 5, 6, 8. Meadfoot Beds, 2, 5, 6, 8. Meadrose, 92. Menniridden, 96. 116 INDEX. Merope Rocks, 19, 89. Metamorphio rocks, 2-5, 12, 15, 17, 19, 27-34, 38, 41-43, 47-51, 63-77, 92, 93, Plates II., lY. Metherin, 96. Mica, analyses, of, 54. schist, 15, 30, 66, 74. trap, 2, 61, 62. Michaelstow, 28, 47, 63, 70, 76, 77, 106. Millpool, 53, 56, 59, 63, 67, 74, 75. Minette, 2, 61, 62. Mining, 1, 99-108. Minnows Island, 11, 23. Minster, 29, 30, 41. Minverite, 46, Plate IV. Molesworth, Wheal, 104. Monazite, 110. Monticuliporid, 27. Mother Ivey's Bay, 10, 19, 20, 89. Mouls, 42, 89. Mountain, The, 27. Mount's Bay, 93. Mundic, lOU. 105. Mutton's Downs, 97. Nehdener Schiefer, 26. Neolithic flakes, 88. Newhouse, 32. Newland, 42, 89. Newquay, 62. New Red Rocks, 62. Newton, E. T., 15. Newtrain Bay, 20. No Man's Land, 12, 13. North Cornwall China-clay Works, 97, 98. Porthilly Mine, 101, 105, 106. Trevarner, 43. Northwood, 97, 98. Norway, ess&ites of, 45, 46. Oatlands Wood, 83. Old Delabole Slate Co., 33. Park 102. Treb'urgett Mine, 105, 106. Trewetha Mine, 105, 106. Onslow Consols, 105. Orkneys, oamptonites of, 46. Orthocerasha.nd, 25, 27, 28. robertsi, 11. Ottrelite, 33, 35, 110. Outground Mill, 65. Overlap of Culm Measures, 34. Overthrusts, 32, 34, 38, 69. Padstow, 1, 3, 13, 22-24, 27, 37, 58, 61, 79, 81, 89, 91, 99, 106, 109, 110; Bay, 93; Consols, 105, 106; Golf Links, 44 ; Harbour, 22, 26,81,93, 107, 111. Pantersbridge, 68, 70. Park Head, 8, 10, 11, 15, 47, 89. Parkinson, J., 4, 28,30, 31, 33, 36, 45, 46, 48, 110. Parkwalls, 51, 78. Park Wood, 61. Pattison, S. R., 3, 31, 35, 87, 109, 110. Pawton Quarry, 11, 91 ; Mine, 12, 100, 101, 105, 106, 110. Peach, 57, 104. Peat, 1, 80, 86-88, 94-96. Penbugle Mine, 106. Pencarrow, 14, 52, 53, 70. Pendarves Island, 8, 9. Pendogett Mine, 102, 105, 106. Pendrift, 75. Pengenna Mine, 102, 105, 106. Penhale Mine, 102. Pennant, 39. Penquean, 23, 81, 82, 91-93. Penrose, 11. Penrose Burden, 69, 75. Pentire, 3, 5, 26, 38, 39, 41-43, 48, 81. Pentireglaze, 25, 42 ; Mine, 102, 105, 106. Pentire Haven, 25. Mine, 102, 105, 106. Steps, 10. Permian, 2, 62. Petherwin Beds, 34. Pewter, 107. Phillips, Prof. J., 109. , J. A., 3, 33, 35, 41, 4.% 45, 110. PhUlipsastraea, 18, 19. Phosphatio nodules, 11, 17, 18, 20, 22. Photographs, list of. 111. Phyllites, 35, 48, 66, 67, Plate IV. Physiography, 1-3, 51, 94, 95. Pillow-lava, 2, 3, 5, 21, 25, 26, 28, 31, 34-39. Ill, Plate IV. Pinite, 58, 59, 66. Pipe Rolls, 99. Pityme, 60. Plateaus, 78-81, 84, 111. Player, J. H., 33. Pliocene, 6, 15, 78-80, 83, 84. Plymouth Limestone, 12. Pneumatolitio changes, 74, 75. Polbrook, 13, 14. Poley's Bridge, 15. Pollard, Dr. W., 46, 62, 104. Polmorla, 83. Polpry, 70. Poltreworgey Mine, 102, 103, 105, 106. Polventon, 89. Polyphant-stone, 91. Polzeath, 25 ; Mine, 102, 105, 106. Portgaverne, 27, 40. Porthcothan, 10, 11, 36. Porthilly, 24. Porth Mear, 10. Porthmissen, 18, 20-22, 37, 89. Port Isaac, 3, 27, 36, 37, 39-41, 102 ; Bay, 79. Port Quin, 26-28, 39, 48. Posidonomya venusta, 25. Freeze, 67i INDEX. 117 Pridaooombe Downs, 88. Proterobase, 2, 19-22, 43-46, Plate IV. Provis, J., 102, 103. Pryce, W., 103. Pteroconus mirus, 8-10, 12. Purple slates, 5, 21, 23, 25-27, 49, 50. Pyrites, 100, 105. Pyritised fossils, 12, 20, 21. Pyromorphite, 104. Quartz-albite veins, 31. — '■ diabase, 2, 10, 42, 47. ■ porphyry, 2, 58-61. Quartzite, 5. Queen Bess Bock, 10. Quies, 89. Eacecourse Downs, 15. Badiolaria, 18, 48. Eadley, E. G.. 33, 41, 45, 46, 54, 62. Rainfall, 108. Eaised Beach, 2, 80-83, 111. Rammelsberg, C. F., 54. . Rashleigh, P., 109. Ratchells, 92. Red ClifE Castle, 10. Reedy ClifE, 39. Raid, Mrs., 26. Rivers, 1. River-ice, 81-83. Terraces, 83-85. Road metal, 47, 93. Robins, Wheal, 102, 104-106. Rock, 22, 24, 43, 44, 48 ; golf-links, 81, 101. Rocky Valley, 30. Roman mining, 99, 107. Rongstook, 45. Roofing-slate, 32, 33, 91-93. Rooke Mill, 25. Roscarrock, 99. Rosenbusch, H., 18, 37, 44-46, 49. Rose Tresungers Mine, 105, 106. Roughtor, 52, 55 ; Mine, 102, 105, 106. Round Hole, 18. Row, 53, 56, 63, 69, 74. Rudler, P. W., 111. Rumps Point, 48, 89. Rutley, P., 35, 110. St. Austell, 54, 70, 74, 96. St, Breook, 81, 102, 106 ; Downs, 1, 5, 79 83 108 St. Breward,'8, 12, 47, 53, 55, 56, 69, 74, 75, 79, 96, 106. St. (Jlether, 31,41,106. St. Dennis, 54. St. Endellion, 40. St. Enodock, 45. St. Eval, 106. St. George's Well, 22. St. Issey, 106. St. Just, 54. St. Kew, 38, 40, 41, 60, 103, 106, Plate IV. St. Mabyn, 15, 28, 39, 40, 43, 44, 45, 61, 62. St, Merryn, 17, 36, 47, 91, 104, IOC. St. Michael's Church, 42. St. Minver, 24, 28, 35, 40, 43, 46, 103. St. Neot, 8, 15, 60, 63, 68, 70, 72, 75, 83, 84, 87, 101, lOfi, 108. River, 51, 98, 100, 103. St. Saviour's Cove, 43. Point, 22, 81. St. Teath, 28, 106. St. Tudy, 39, 40, 75. Salem, 46. Samaritan Island, 10. Saiid-banks, 88, 89, 107 ; dunes, 82, 88,89, 111. Sandstone, 5-8. Scarnor Point, 39. Scenery, 89, 95, 111. Schalstein, 2, 37, 40. Schist, 31, 47, 48, 69, 74, Plate IV. Schorl, 52, 53, 55-57, 61, 67, 74, 75, 96, 104. Scilly Isles, 57, 78. Sedgwick, Prof. A., 3, 109. Septaria, 18. Sharpe, D., 3, 109. Shed-tin, 87. Shelf, 86, 94. Shell-sand, 3, 109. Shell Wood, 59. Shore-lines, ancient, 78-80. Shorters; 92. Silica, soluble, 7. Silver, 99, 102-105. Sisters, Wheal, 102, 105, 106. Sladesbridge, 39, 42, 43. Slaughterbridge, 29, 30, 63, 65, 68, 84, 92. Slate-dressing, 111 ; quarries. 2, 23, 91-93. Slater, Miss I., 18. Small's Rock lighthouse, 90. Smithson, J., 109. Soda-rock, 19, 49. Soil, 2. South Petherwin, 17, 34. Spilite, 2, 40, 41, Plate IV. SpUosites, 48-50. Spirifer verneuili, 30, 91. Spittal, 15. Spotted slates,' 48-50. Staddon Grit, 2, 5-8, 11-13, 79, 81, 93, 108. Standing stones, 5, 6. Stannaries, 111. Stannon Marsh, 68, 80, 95, 97, 98, 107, 108. Stepper Point, 2, 22, 42, 43, 49, 50, 81, 89, 91, 93, Plate H. Stock- work, 103, 104. 118 INDEX. Stone circles, 5, 6. quarries, 2, 90, 91. Stoptide, 25. Strain-cleavage, 13, 14, 16, 66. Strata, table of, 2. Stream-tin, 1, 80, 83, 85-88, 99, 107, 108, 111. Strike, 2, 3, 12, 31, 32, 92. Stuliola, 18, 20, 23, 24, 26. Submarine lavas, 35, 38. Submerged forest, 82, 109. Sunnycoruer, 37, 81. Sympterura minveri, 1 10. Synclines, 2, 25, 32. Tamar, River, 1. Tamill, 31. Teall, Dr. J. J. H., 3, 35, 45, 49, 110. Temple, 59, 94, 96, 97, 108. TentacuUtes, 11, 17, 26, 34. Tertiary, 78-80, 93, 111. Thermometamorphism, 63-77. Thomas, H. H., 71, 111. Dr. T., 11, 12, 25, 26, 111. Thorne, 53. Thrust-planes, 7. Tichbarrow, 29, 30, 47. Tin-lodes, 56-58, 75 ; mines, 1, 99-108 ; stream, 1, 83, 85-88; 107, 108, 111 ; yield of, 99. Tintagel, 1, 4, 17, 28-30, 32, 33, 36, 42, 79, 80, 92, 106. Tooke, A. W., 103. Topaz, 53, 60. Tors, 51-53, 56, 57, 78, 111. Tourmaline, 52, 53, 55, 56, 57, 61, 67, 68, 74, 75, 96, 104. Towans, 88, 89. Tower Bridge, 90. Trambley Cove Beds, 28, 29, 31, 33, 34. Trebarwith, 28-33, 78, 79. Trebetherick Mine, 102, 105, 106. -■ Point, 25. 42, 81. 82, Plate n. Treboy, 47. Treburgett Mine, 102, 105, 106. Treburrick, 11. Trecarne, 68, 70, 73. Tredole, 31. Tredorn PhyUites, 28, 29, 31-34. Tredrizziok, 25. Trees in peat-mosses, 87. Treforda, 31 Trefreaa Mine, 103, 105, 106. Trefrew, 65. Tregaddock, 44-46. Tregaina, 31. Treganna, 24, 48, 88. Tregardock, 27, 28, 79 ; Mine, 103, 105, 106. Tregatherel, 31. Tregatta, 31. Tregeagle Mine, 70, 101, 103-106. Tregelles Mine, 103, 105, 106. Treglasta, 29, 31. Treglennick, 47. Treglyu Mine, 103, 105, 106. Tregonce, 22. Tregorden Mine, 103, 105, 106. Tregrylls, 31. Tregue, 47. Tregullan, 71. Tregulland, 31. Tregunna, 23. Treharrock, 40. Trehudreth Mill, 67. Treknow, 30, 31 ; Mine, 103, 105, lOfr. Treligga, 30-32. Trelights, 39. Trelill, 35, 41, 102. Trelow Mine, 103, 106. Tremaddock, 68. Tremail, 28. Tremblary, 31. Trenale, 79. Trenance, 5, 89. Trenuth, 65. Treore, 40. Trequite, 28. i^rGncG 13 Treroosel Mine, 103, 106. Trerubies Cove, 27. Tresallyn, 36. Tresarrett Wood, 60. Trescore Island, 10. Treslea Downs, 60, 68. Tresungers Mine, 103, 106. Trethevy, 31. Trethin, 72, 73 ; Mine, 103> 106. Trevalga, 29, 31, 33. Trevan Head, 26. Trevanson, 23. Trevarra Mine, 103, 106. Trevebban, 11. Treveddoe, 75 ; Mine, 103-106. Treveglos, 17 ; Mine, 104, 106. Treveighan, 47. Trevena, 32, 42. Trevilgus, 36. Trevilledor, 6. Trevillick Mine, 104, 106. Trevinnick Mine, 104, 106. Trevisker, 36. Trevivian, 30, 47. Trevone, 20,21, 33, 43, 81. Trevorgus, 11, 36, 104. Trevorrick, 81. Trevose Head, 19, 48, 49, 89. Trewane Mine, 104, 106. Trewardale, 59. Trewassa, 32, 33, Plate IV. Trewetha, 40 ; Mine, 104, 106. Trewethern, 60. Trewethet, 30. Trewins, 31. Trewint Bill, 45. Treworder, 18, 43. Trewornan House, 23. INDEX. 119 Treyarnon, 18, 36. Trezelland Mine, 106. Tuff, 2,6, 8, 11, 17, 22-25, 28, 35-37, 39,40, HI. Tumrose, 67. Tungsten, 1, 2, 80, 85-88, 107, 108, 111. Tyland's Quarry, 70. Umber, 103. Ussher, W. A. E., 17, 110, HI. Valley-erosion, 78-81, 83-87, 99, 111 ; Gravel, 2. Varley Head, 27. VUIaparks, 30, 32, Plate IV. Volcanic rocks, 2, 3, 5, 8, 11, 17, 22, 28, 29, 31, 34-42, 109-111. Wack6, 3. Wad 39. WadeMdge, 1, 11-13, 22, 23, 27, 28, 43, 48, 58, 60, 83, 91, 99, 107. Walker, Miss G., 26. Warleggon, 15, 60, 63, 68, 70, 75, 84, 97, 104, 106. Washington, H. S., 111. Wastrel, The, 40. "Water supply, 108. Waterpit Downs, 29, 30, 32. Watersheds, 1. Webb & Geach, Messrs., 100. Wells in granite, 51. Wenfordbridge, 14, 15, 37, 63, 68, 73, 97, 98. Wheal, see name of mine. Whidborne, Rev. G. F., 9. Whisper, Wheal, 103, 104, 106. Whitecross, 102. Whitewell Mine, 104, 106. Whitley, N., 3, 109. Williams, Rev. D., 35. Wolfram, 1, 2, 80, 85-88, 107, 108, 111. Woodlands, 2. Woodward, Dr. A. S., 9. Dr. H., 4, 10, 20, 24, 110, 111. Woolgarden Phyllites, 28-30, 32-34. Worthyvale, 47, 48, Plate IV. Zinc, 99, 102, 105. Zoisite, 71. Zones, palseontological, 9, 12, 17. 120 PLATE I. Banded purple and green slates at Trebetheriok Point. The light undulating lines are the edges of the inclined green slates, while the dark spaces between are the purple beds. At certain places, in softer beds of these variegated slates, little ostracods (^Entomis serratostriata) and a lamelli- branch {Posidonomya venusta) are very numerous. The foreshore is the gently-shelving raised beach platform (seen in profile in the middle distance), and is covered locally with a pebbly beach. Cemented blown-sands constitute the low cliffs which here and there are capped with a boulder bed, seen above the figure. The distant ridge is the pillow lava which forms Pentire Head. PLATE IL Illustrating the relation of the Greenstone Dyke of Stepper Point to the surrounding rocks. Behind the hammer is a narrow sill or spur given off from the great vertical dyke (a few yards to the right of the photograph). This spur coincides approximately with the bedding of the Upper Devonian shale, but distinctly cuts across it just below the hammer. The shale is much altered and spotted, white hollow nodules having developed along the bedding planes. PLATE III. Roughtor. Showing characteristic of a tor formed in the sounder granite (see p. 51). PLATE IV. Photomiceogkaphs. Pia. I. — Proterobase (Minverite), Rock Quarry, St. Minver (E 5596, magnified 17 diameters ordinary light). In this view the typical intergrowth between angite (central) and hornblende (peripheral) is clearly shown. Both minerals are earlier than the large clear albite felspar in which they are embedded. Fig. II.— Albite Diabase, Kit Hill Quarry, St. Minver (E 5613, magnified 13 diameters ordinary light). This photograph shews augite and albite felspar crystallised out from an eutectic mixture simultaneously and partially enclosed in later ilmenite. Fig. III. — Sheared granitic rock, Worthyvale, Camelford (E 6517, magnified 13 diameters, polarized light). Shews augen structure produced by shearing. The " eyes " are albite felspar and quartz and are wrapped round with actinolite and chlorite fibres. Fig. IV. — Spilite, north-east of Lanow, St. Kew (E 5620, magnified 11 diameters, ordinary light). This is a section of 'pillow lava,' shewing its highly vesicular nature. The groundmass is partially devitrified and contains small rod-shaped albite felspars. The vesicles are filled with calcite and chlorite. Fig. V. — Garnetiferous phyllite, Villaparks, Camelford (E 6514, magnified 16 diameters, ordinary light). The garnets contain isotropic grains arranged in various patterns such as chevrons, crosses and stars. The sericite of the ground - mass has been parted by the garnets during their growth. Fig. VI. — Sericite-chloritoid schist, Trewassa, Davidstow (E 6532, magnified 29 diameters, ordinary light). The photograph shews chloritoid crystals with hour-glass structure, and white mica growing across the cleavage planes of the phyllite. 15999 Geology of Padstow and Camelford. Plate II. H B o o Ph bo o O 16999 K Geology of Pauistow ^ Coarudford PUvb&W. 3. 5. e. PHOTOMICROGRAPHS OF ROCK SECTIONS. IfuthfLondon. 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