v_ fyxmll Hmvmitg |fib»g BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF Henrg W. Sage 1891 ftj.Qi>.7a8: fflmtf/& 3777 Cornell University Library QE 262.P73U87 1907 The geology of the country around Plymou 3 1924 004 552 067 Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924004552067 MEMomS OF THE GEOLOGICAL SURVEY. ENGLAND AND WALES. EXPLANATION OF SHEET 348. THE GEOLOGY OF THE COUNTKY AROUND PLYMOUTH & L1SKEAKD. BY W A. E. USSHER, F.G.S. WITH NOTES ON THE PETROLOGY OF THE IGNEOUS ROCKS BY J. S. PLETT, M.A. D.Sc. PUBLISHED BY ORDER OF THE LORDS COMMISSIOHEES OF HIS MAJESTY'S TREASURY. LONDON: PRINTED FOR HIS MAJESTY'S STATIONERY OFFICE BY WYMAN & SONS, LIMITED, FETTER LANE, E.O. And to be purchased from E. STANFORD, 12, 13 and 14, Long Acre, London ; Ltd., W. & A. K. JOHNSTON, 2, St. Andrew Square, Edinburgh ; , HODGES, FIGGIS & Co., Grafton Street, Dublin. From any Agent for the sale of Ordnance Survey Maps ; or through any Bookseller from the Ordnance Survey Office, Southampton. 1907. Price 3s. .. ; LIST OF MAPS, SECTIONS, AND MEMOIRS OF THE GEOLOGICAL SURVEY OF ENGLAND AND WALES, AND MUSEUM OF PRACTICAL GEOLOGY. J. J. H. TEALL, M.A., D.Sc, F.R.S., Director of the Geological Survey and Museum, Jermyn Street, London, S.W. The Maps and Memoirs are now issued by the Ordnance Survey. They can be obtained from Agents or direct from the Ordnance Survey Office, Southampton. Museum Catalogues, Guides, &c, are sold at the Museum, 28, Jermyn Street, London. A Complete List of the Publications can be obtained from the Ordnance Survey Office, Southampton. Price M. , INDEX MAP (25 Miles to the inch). Map of the British Islands. Price—Coloured, 2s., uncoloured, Is. GENERAL MAP (one inch to 4 miles). ENGLAND AND WALES.—Sheet 1 (Title) ; 2 (Northumberland, &c.) ; 3 (Index of Colours) ; 4 (I. of Man) ; 5 (Lake District) ; 6 (E. Yorkshire) ; 7 (North Wales) ; 8 (Central England) ; 9 (Eastern Counties) ; 10 (South Wales and N. Devon) 11 (W\ of England and S.B. Wales) ; 12 (London Basin and Weald) ; 18 (Cornwall, &c.) ; 14 (South Coast. Torquay to I. of Wight) ; 15 (S. Coast, Havant to Hastings). Sheet 1, 28. : sheets 2 to 15, 2s. 8<J. each. Printed in colours. NEW SERIES.—Drift, Sheet 8 (Flamborough and Grimsby), 2s. ; Sheet 12 (Peterborough, Norwich), is. 6<t Solid, Sheet 1 and 2 (Alnwick, Berwick, etc.), price 2s. 6ii. ; Sheet 4 (Newcastle-upon-Tyne, etc.), 2s. 6d. MAPS (one-inch). Old Series. Nos. I to 110 in whole sheets and quarter-sheets, hand-coloured, in two editions, Solid n nd Drift—except 92 NE., SE., 93 NW., are published Solid only. SW., 97 SE., 98 NW., SW., SE., 101 SE., which Prices, whole sheets, is. to 8s. M. ; quarter- sheets, Is. 6d. to 3s. Sheet 7, Drift, 18». 6d,. MAPS (one-inch). New Series, 1 to 73. These New Series maps are identical with the Quarter Sheets of the Old Series, Nos. 91 to 110, all of which are published with the Drift, excepting Sheets 29, 38, 48, 49, 51, 61, 62, 69 and 70. Price. Price Solid. Drift. Solid. s. d. s. d. LONDON in four Sheets (Colour printed) 317. CHICHESTER (Colour printed) .. each 1 6 325. EXETER ISLE OF MAN Sheets 36, 45, 46, 66 and 67 8 6 8 6 326 and 340. SIDMOUTH and LYME 110. MACCLESFIELD (Colour printed) — I REGIS (Colour printed) .. 123. STOKE - UPON - TBENT (Colour 328. DORCHESTER (Colour printed) .. printed) 1 6 329. BOURNEMOUTH, WIMBORNE 141. LOUGHBOROUGH (Colour printed) (Drift, Colour printed)^ . ..80 155. ATHERSTONE 330. Parts of NEW FOREST and part of 166. LEICESTER (Drift, Colour printed) ISLE OF WIGHT (Drift, Colour printed) -, .. ..3 187. HUNTINGDON 331. PORTSMOUTH and part of ISLE 203. BEDFORD OF WIGHT (Drift, Colour printed) 8 280. AMMANFORD (Colour printed) . ISLE OF WIGHT (Colour printed) . — 231. MERTHYRTYDFIL(Colourprinted) 332. BOG NOR, &c. (Colour printed) ..16 232. ABERGAVENNY (Colour printed) 333. WORTHING, ROTTINGDEAN .. — 246. WORMS HEAD (Colour printed) . 334. NEWHAVEN, EASTBOURNE (Colour 247. SWANSEA (Colour printed) printed) — 248. PONTYPRIDD (Colour printed) 339. NEWTON ABBOT — 249. NEWPORT (MON.), (Colour printed) 341. WEST FLEET (Colour printed) .. — 261 and 262. BRIDGEND (Colour printed) 842. PORTLAND, WEYMOUTH, LUL- WORTH (Colour printed) .. .. — 263. CARDIFF and WORLE, SOMER- SET (Colour printed) .. 343. SWANAGE.CORFE CASTLE (Colour printed) — 267. NEWBURY (Colour printed) .. 846. NEWQUAY (Colour printed) „ .. — 268. READING (Colour printed) .. 348. PLYMOUTH and LISKEARD (Colour 282. DEVIZES (Colour printed) .. printed) — 283. ANDOVER (Colour printed) .. 349. PLYMOUTH and IVYBRIDGE .. — 284. BASINGSTOKE (Colour printed) .. 350. TORQUAY — SALISBURY (Colour printed) 351 and 358. LAND'S END DISTRICT WINCHESTER (Colour printed) .. (Colour printed) — 300. NEW ALRESFORD (Colour printed) 352. FALMOUTH & TRURO (Colour printed) — 811. WELLINGTON and CHARD 863. MEVAGISSEY (Colour printed) .. — (Colour printed) 365. KINGSBRIDGE 814. RINGWOOD (Colour printed) 356. START POINT 315. SOUTHAMPTON (Colour printed). 367 and 360. ISLES OF SCILLY (Colour 816. HAVANT (Colour printed) .. printed) II 8 CD g s Q W w o I— « > I— wCO H J<i 02 O > a fi H a o o 3 w S <! OEl, H X w « M -3 aD MEMOIRS OF THE GEOLOGICAL SURVEY. ENGLAND AND WALES. EXPLANATION OF SHEET 348. THE GEOLOGY OF THE COUNTRY AROUND PLYMOUTH & L1SKEAKD. BY W A. E. USSHER, F.G.S. WITH NOTES ON THE PETKOLOGT OF THE IGNEOUS ROCKS BT J. S. FLETT, M.A. D.Sc. PUBLISHED BY ORDER OF THE LORDS COMMISSIONERS OF HIS MAJESTY S TREASURY. LONDON: PRINTED FOR HIS MAJESTY'S STATIONERY OFFICE BY WYMAN & SONS, LIMITED, FETTER LANE, E.C. And to be purchased from E. STANFORD, 12, 13 and 14, Long Acre, London ; W. & A. K. JOHNSTON, Ltd., 2, St. Andrew Square, Edinburgh ; HODGES, FIGGIS & Co., Grafton Street, Dublin. From any Agent for the sale of Ordnance Survey Maps ; or through any Bookseller from the Ordnance Survey Office, Southampton. 1907. Price 3s. E..V. ft.36ftT2.'8 Ill PREFACE. The original geological survey of this area by De la Beche was published during the years 1834 and 1835, almost wholly on Sheets 24 and 25 of the old series one-inch maps. These " sheets were revised in 1839, when the " Carbonaceous series " was separated from the Grauwaeke series " ; and about the year 1846 (or 1845) the classification and colouring were modi- fied. De la Beche then adopted the term Devonian Rocks, and stated on the index of colours that they were represented by " A provisional colour, as there is reason to consider that the rocks included under this head in Cornwall and Devon are equivalents of Carboniferous Limestone, Old Red Sandstone, and Upper Silurian Rocks." The igneous rocks and the Devonian limestones were separated from the mass of slates and grits on De la Beche's map, but the succession of the rock- groups, and the intimate structure of the area had not been determined. In his monumental ' Report on the Geology of Cornwall, Devon and West Somerset,' published in 1839, De la Beche described the leading types of rock, and made many sugges- tions with regard to their correlation that have not stood the test of later detailed researches. Nevertheless, his facts were noted with such precision that the present author, from experience he had gained in the Devonian area of South Devon, was enabled so to interpret De la Beche's observations as to be able to give in 1890 a forecast of the probable structure in the Devonian rocks of Cornwall. Mr. Ussher then represented on a small map of that county the equiva- lents of the main divisions which with great care and labour he had established in Devonshire. It is interesting to find that the great structural feature in the Plymouth area, the faulting that displaces the Lower Devonian rocks and brings them for some distance against Upper Devonian, was depicted on that map of 1890, and that the main sub-divisions, which Mr. Ussher has established in Devonshire, have been confirmed Geological by the detailed work since carried out by the Survey. , The area represented by this map was surveyed on the six- memoir. inch scale by Mr. Ussher, who has also written the by The petrology of the igneous rocks has been dealt with has contributed Dr. J. S. Flett, and Mr. D. A. MacAlister of particulars relating to mines and minerals. The author progress of the memoir has given an historical account of the compiled geological knowledge in the area, and he has also a bibliography which is printed as an appendix. J. J. H. TEALL, Director. Geological Survey Office, 28, Jermyn Street, London. 7th October, 1907. — IV NOTE. The following is a list of the six-inch Geological Maps included in the area, of which MS. coloured copies are deposited* for public reference, in the Library of the Geological Survey and Museum of Practical Geology : ———— CONTENTS. PAGE Preface by the Director iii Chapter I.—Introduction. Literature — Tables of Formations — Cartography — General Geology and Structure 1 Chapter II. Lower Devonian. General Structure ..• 18 Dartmouth Slates ... 21 Meadfoot Group.. 27 Staddon Grits 40 Igneous Rocks in the Lower Devonian ... ... 46 Chapter III. Middle Devonian. Plymouth District 50 St. Germans District ... 63 Liskeard District 64 Igneous Rocks in the Middle Devonian 69 Petrography of the Igneous Rocks, by Dr. J. S. Plett 70 Chapter IV.—Upper Devonian. General Notes —Types, Fossils— Association with volcanic rocks—Distribution of colouring matter—Fossil Localities ... 73 The Wearde-Efford type 82 Igneous Rocks in the Upper Devonian 90 Petrography of the Igneous Rocks, by Dr.