(The Speeies Figured in This Plate Have All Been Obtained ~Tom Holwell. ) Fig. 1. Trochus Holwellensis, Spcc. Nov. : Back View. 2
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Downloaded from http://jgslegacy.lyellcollection.org/ at New York University on January 17, 2016 568 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. PLATE X-VII. (The speeies figured in this plate have all been obtained ~tom Holwell. ) Fig. 1. Trochus Holwellensis, spcc. nov. : back view. 2. --~ : base, enlarged. 3. 1VeritoTsis l~vis, Stol. : front view. 4. -- -- : top view. 5. Trochus gradatus, spec. nov. i enlarged. 6. _Phasianella turbinata, Stol. : back view. 7..4mberle~/a aITina, Stol. : front view. 8. DeIThinula reflexilabrum, Home: front view. 9. --- -- : top view. 10. -- nuda, spec. nov. : back view. 11. Troehus latilabrus ?, Stol. : back view. 12. IYerin~a Horneri, spec. nov. : front view. 13. Pleurotomaria Buchi, Desl. : front view. 14. -- -- : surface view. 15. Solarium lunatum, spec. nov. : top view. 16. Turbo nodulo-carina~us, spec. nov. : back view. 17. -- ~ : front view. 18. solidus, spec. nov. : front view. 19. Avicula nuda, spcc. nov. 20. Opis triangularis, spee. nov. : dorsal surface. 21. Lima DeslongchamTsi, Stol. 22. scrobicula4a,Stol. 23. - den~icosgata, Quenst. ~A. ---- 25. Pecten Rollei, Stol. 26. verticillus, Stol. 27. Talosus, Stol. 2. On the PHYSICAL STrUCTUrE of WEST S0~rERSET and NORTH Dv.V0N, and on the PAL~ONTOLOOICAL VALVE of the DEVONIAN :FossILs. By ROB~RT ET~RIDGE, Esq., F.G.S., F.R.S.E., Palmontologist to the Geological Survey of Great Britain. CONTENTS. I. Introduction. IV. Structure and Succession of the II. History and Literature. Rocks of North Devon. III. Structure and Succession of the A. Lower Devonian Group. :Rocks of West Somerset. 1. Porlock to Iffnton. 1. Cannington Park Limestone. 2. Foreland, east of the River 2. The Quantoek Hills. Lynn. 3. Dunster, Minehead, and Port- 3. Woodabay and Heddon's lock Areas. Mouth. 4. Dunster to Dulverton. 4. Conclusions relative to the 5. Valley of the Barle and west Lower Devonian Series of of Dulvert0n. West Somerset and the Lynton 6. South of Dulverton. area. 7. Dulverton to Wiveliscombe. B. Middle Devonian Group. 8. Dunster to Brendon Hills and 1. Heddon's Mouth to Barn- 1%ttlecombc. staple. 9. Withycombe by Dunster. 2 Ilfracombe to Baggy Point 10. Goldsoncot. .rid Morte Bay, Little Hang- 11. Higher Roadwater, Trap Hole, man Hill, and West Challa- Lod Huish. combe to Ilfracombe. Downloaded from http://jgslegacy.lyellcollection.org/ at New York University on January 17, 2016 ]~THERIDGE--DEVONIAN ROCKS AND FOSSILS. 569 3. Helesborough. South Devon, and West So- 4. Ilfracombe District. merset. 5. Lee and M:orte Bay. 6. Agreement of Foreign Devo- 6. Conclusions relative to the nian species with the British Middle Devonian or Ilfra- Middle and Upper Series in combe Group of West Somerset North and South Devon. and 1%rth Devon. 7. Analysis of the British and V. Palmontologieal Value of the Or- Foreign Devonian Brachio- ganic Remains found in the poda. Devonian Groups. 8. British Devonian and Car- 1. Chronological Equivalents of boniferous Brachiopoda. the Old Red Sandstone of VI. Stratigraphical Value of the Spe- Hereford, Scotland, &e. cies comprising the Devonian 2. Lower Devonian Fossils of Fauna. West Somerset and 1%rth 1. Amorphozoa. Devon. 2. Ccelenterata. 3. Fossils of the Middle Devo- 3. Crustacea. nian or Ilfracombe Group. 4. Cephalopoda. 4. Species common to the Middle 5. Brachiopoda. Devonian Rocks of 1%rth 6. Other Classes. and South Devon. VII. Stratigraphical considerations on 5. Species common to the ]~Iiddle the Devonian Fossils. Devonians of 1%rth Devon, I. INTRODUCTI01~. IT seems almost superfluous that another paper should be written upon the structure and succession of the Rocks of "West Somerset and North Devon;" but of late their position or place in the geological series has been questioned in an able paper by Pro- fessor 5ukes ~, in which he endeavoured to prove that the entire series of slates, sandstones, and limestones of the North Devon and West Somerset area belong partly to the 01d Red Sandstone and partly to the Carboniferous rocks, rather than to the so-called Devo- nian Group, to which they have hitherto been considered to belong. In other words, Professor Jukes has propounded views as to the re- lative succession and physical structure of North Devon diametrically opposed to those held by most geologists, and based upon the investi- gations of Sir Roderick ~[urehison and Professor Sedgwick, Professor John Phillips, De La Beche, Weaver, and others ; and he distinctly states that they have "all misunderstood the structure of the country," and this arising chiefly from their having been previously unacquainted with the structure and succession of the lower mem- bers of the Carboniferous group of rocks, and with the upper series of the Old Red Sandstone, as shown in the South of Ireland (the s Old Red Sandstone). Professor Jukes also disputes the reality of the "time "succession (not the apparent) of the rock-groups of North Devon, i. e. those grits, slates, and sandstones which occur (in the Lynton area) from Lynton or the Foreland, on the north, to the base of the PickweU Down or ~orte sandstones, on the south, or more extendedly to Barnstaple, which-is the extreme southern part of the l~orth Devon * quart. Journ. Oo61. Soc. vol. xxii. p. 321. August 1866. Downloaded from http://jgslegacy.lyellcollection.org/ at New York University on January 17, 2016 570 pROC~.EDII~6SOF THE GEOLOGICAL SOCIETY. group under examination. This area is occupied by varied rock masses (three well-defined groups), consisting at the base of red sandstone and slates with gritty and subcalcareous bands, which form the Lynton or Lower Devonian series; succeeding these are red sandstones and slates, well-defined calcareous masses, and bands of limestone, which constitute the Middle Devonian or Ilfracombe group ; these are surmounted by a thick zone of red sandstones (the Upper Old Red), which are again overlain by a great series of slaty subcrystalline arenaceous limestones and shales, brown sandstones and grits, which constitute (with the red sandstones below) the Upper Old Red Sandstone or Upper Devonian Series, partly the equivalents of those beds which occur in the Cork district, and termed by the Irish geologists Carboniferous Slate, Coomhola Grits, and Old .Red Sandstone. It will be my object to endeavour to show that this succession in North Devon and West Somerset is one unbroken and continuous series, existing in both districts, and proved by physical as well as pal~eontological data. It will also be my endeavour to prove that the hypothetical fault which Professor Jukes has stated to range from Morte Bay on the west to Wiveliseombe and the Quantock Hills on the east has no existence through the centre of North Devon,--and also to show that there is no evidence of a concealed anticlinal (of the nature demanded) with a northern in- version, through the agency of which, and the concealed fault with a supposed northerly downthrow of some 4000 or 5000 feet, we are asked to believe that the Lynton sandstones, grits, and slates, and the Middle or Ilfracombe Series in the north part of the county, are upon the same general horizon as those (similar, not identical) rock- masses comprising the structure of Baggy Point, Marwood, Sloly, Croyde, Braunton, Pilton, &c., on the south, and above the Pickwell Sandstones. From these views I entirely dissent on the grounds above stated ; and a careful examination of the West Somerset and North Devon areas during the past autumn (1866) now enables me to lay before the Society the results of that investigation. II. HISTORY A~D LITER&TURE. It is necessary to my purpose that a condensed history of the literature of the Devonian Rocks, with their groupings &c., should be understood, and that our British series, with their continental equivalents and their correlation generally, should be paralleled; we shall thus he enabled to trace the history of the question, and aided in coordinating the views held by those geolo~sts and pakeontologists who have, both in this country and on the Continent, since the year 1837, adopted the nomenclature proposed by Murchison and Sedgwick in their elaborate memoir upon "the Physical Structure of North Devon and the Rhenish Provinces"*, in which they en- deavoured to assign a position and sequence to those slates, sand- stones, and limestones which are so conspicuously and extensively developed in North and South Devon and Cornwall. Their views * Geol. Trans. vol. v. 1840; and Proceedings, vol. iii. 1839. Downloaded from http://jgslegacy.lyellcollection.org/ at New York University on January 17, 2016 v.TH~.~DG~--~)EVO~A~ ROCXS AN~) P0SS~T.S. 571 were ably substantiated by Mr. Lonsdale, by his careful investiga- tion of the fossils, to which he assigned an intermediate and distinct position (to the corals, &e.), showing, with the authors of that memoir, that the stratified rock-masses of that extensive area and their fossil contents hold a position, physically and pal~eontologically, between the great mass of the Silurian. deposits, already definitely determined, and those of the Carboniferous series succeeding them, which are developed on a grand and extensive scale in the culm- trough that occupies the region between Barnstaple, in North Devon, on the north, and Brocastle and Petherwin, in North Cornwall, on the south. 1836. SedgwicIc and Murchison.mThese authors, in a communi- cation to the British Association ~, were the first to accurately de- termine the place of the carbonaceous deposits of North and Central Devon, which had previously been classed with the lowest portions of the Grauwaeke; they described the structure of the county in ascending order, and divided the whole series into five groups ; they, however, then misunderstood the group of rocks that repose upon the ~[orte slates and sandstones in North Devon, referring the Upper Devonian or Barnstaple series to the Silurian system, and placing the Carboniferous upon them, which in stratigraphical succession was correct, the underlying group being referred to a wrong age.