[Plates X. & Xi.] I Introduction

Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on June 28, 2016 ~54 DR. B~. HICKS ON THE HORTE SLATF-~, AND [Afay I895 , 13. On the MoRr~ SLATSS, and ASSOCIATED BEDS, ~n :[70RTK :DEvoN and WEST SOMERSET.--Part I. By H~.~ar HICKS, M.D., F.R.8, P.G.S. (Read :February 5th, 1896.) [PLATES X. & XI.] CONTENTS. Page I. In~roduetion ........................................................................ II. Morthoe and Woolacombe to Bittadon..: .................................... ~57 III. R~kham Bay, Bull Point, Lee Bay, Lee, and Slade .. ................... 261 IV. Mullacott, Shelfln, and Ilfracombe .......................................... 262 V. Woolseott Barton, S,nithson, and Berry Down ........................... 264 VI. Summary of the Stratigraphical Evidence in North Devon ............ 264 VII. Description of the Fossils found in North Devon ........................ 266 Geological Map of the Ilfracombe and Morthoe Districts ............... 25~} I INTRODUCTION. ]:[IT~RTO the Morte Slates have received but scant attention from geologists, owitJg to the fact that in all text-books, and in. geological papers on North Devon, they have always been referred to as unfossiliferous ; and all that was known with regard to them u~ to the year 1887 is well summarized in the following paragraph taken from the second edition of Mr. H. ]3. Woodward's well-known work, ' The Geology of England and Wales,' p. 127 :--' This division, termed the )[orte Slates by the Rev. D. Williams, derives its name from Morte Point, on the north-west coast of Devon. The term Mortehoe Group, from the village of that name, was used by John Phillips. The Morte Slates, or " grey slates," comprise pale greenish-grey aud silvery grey glossy slates, much veined with quartz, and having a thickness estimated at from 3000 ~;o 4000 feet. No fossils have been found ; nor have any limestone-bands been recognized in them. The beds rest on the Ilfraeombe Beds at Lee Bay, and the subdivisions which can be traced are noted by Mr. :Etheridge (in ascending order) as the Lee, Rockham Bay, and Morthoe Beds. The Morto Slates pass downwards into the Ilfracombe Beds, and in Mr. Ussher's opinion they are simply an upper unfossiliferous portion of this lower division, since it is impossible to fix any definite boundary between them. Simonsbath is situated in the valley of the Barle, between the Ilfracombe and Morto Beds. Eastwards they extend to near Wiveliscombe, where they are exposed at the Oakhampton Slate quarry, north of that town. The blaty beds of ttestercombe, north of Taunton, are probably on the horizon of the .~orte Slates. The valuable spathose iron-ore of the Brendon Hills occurs in these beds.' Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on June 28, 2016 u 52.] ,SSOC~A~DBEDS, IN N. DEVON A~D W. SOMERS~V. 255 On November 26th, 1890, I read a paper before this Society ' On the Rocks of North Devon,' in which I stated that during a recent visit to North Devon not only had I found that the Morte Slates were fossiliferous, but that I had come to the conclusion that they were the oldest rocks in the area, and that they had, as the result of movements in the earth's crust, been brought to the surface and thrust over much newer rocks, producing a deceptive appearance of overlying the latter conformably. As my views regarding the succession were strongly controverted at the time, the paper was withdrawn, and I decided to re-examine the area, and to carry my researches into other districts which I had not previously had an opportunity of visiting. As this has necessitated the spending of several weeks each year, since the paper ~vas read, either in North Devon or West Somerset, much additional evidence beariug on the succession has been obtained; but in this that relating mainly to the position and age of the Morte ~lates will be referred to. The beds included under the term ' Morte Slates ' are for the most part much folded and highly cleaved, and the fossils are in con- sequence frequently much distorted and in a had state of preservation. A considerable amount of labour and time has therefore had to be expended on these rocks to obtain anything like a satisfactory fauna. The results on the whole, however (when it is remembered that upto 1890 these beds were always referred to as unfossiliferous), must be considered as highly important, since we are now able by the aid of the fossils to define the age of a very considerable portion, if not of the whole, of these beds. That they must necessarily furnish the clue by which the succession in North Devon is to be unravelled will, I think, be generally admitted. Therefore, if their geological horizon can be settled the main difficulties which have so long surrounded lhe ' Devonian question' in North Devon, and which have led to so much controversy in the past, will virtuslly disappear. In the geological map given in Mr. Etheridge's very important paper published in this Journal in 1867, the Metre Slates are shown as extending continuously from :Metre Point to near Wiveliscombe, in West Somerset, a distance of over 40 miles. That rocks which possess characters resembling in many ways the typical Metre Slates are to be found all along this line there can be no doubt, but it ~vill be shown that, here and there, they vary much in appearance and belong to different geological horizons. The strike of the beds also changes, and is seldom quite parallel with the overlying strata. In my search for fossils in these rocks I have received invaluable assistance from my friends the Rev. G. F. Whidborne, M.A., F.G.S., and Mr. J. G. Hamling, F.G.S., of Barnstaple. I have also to express my indebtedness for special assistance with regard to some of the fossils from Mr. Sharman and Mr. Allen, of the Museum of Practical Geology; Mr. Bather, of the British Museum (Natural Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on June 28, 2016 256 l)a. H. aXCKS 01~ xmm Momm Sr.A~ES, Alva [May 1896 , ~History) ; Mr. F. Cowper Reed~ of Cambridge ; 5fr. $. Hopkinson~ and Prof. C. Lapworth, ~ 1 [A complete r&~um~ of the literature of the North Devon rocks up to the year 1867 has been given by Mr. Etheridge in his paper ah-eady referred to. Up to the year 1868 the views put forward by Sed~wick, Murchison, Godwin- Austen, De la Beehe, and Phillips received general ~eptance, but during that and subsequent years Prof. Jukes suggested modifications which tended towards a very different interpretation. In his paper in the Quart. Journ. Geol. Soc. in 1866, vol. xxii., he says at p. 821 : ' As I shall have to maintain that all the first geologists of the day,'ineluding Prof. Sedgwick, Sir R. I. Murehison, Mr. Weaver, Sir H. De la Beche, and Prof. Phillips, have misunderstood the structure of the country, let me hasten to avow my belief that nobody whose observations were confined to Devon and Somerset could have arrived at any other than their con- clusions. I fully admit that the rocks near Lynton appear to be the lowest, and that there appears to be a regular ascending succession of rock-groups from Lyntonto the latitude of Barnstaple. ]~ am, however, compelled to dispute the reality of this apparent order of succession, and to suppose that there is either a concealed anticlinal with an inversion to the north, or, what I believe to be much more probable, a concealed fault running nearly east and west through the centre of North Devon, with a large downthrow to the north, and that the Lynton beds are on the same general horizon as those of Baggy Point and Marwood.' In 1867 Mr. Townshend M. Hall, whose researches have added so much to our knowledge of the North Devon rocks and their fossil contents (Quart. Journ. Geol. Soc. vol. xxiii, p. 371), subdivides the North Devon series, in ascending order, as follows :--Foreland group, Lynton zone, Martinhoe beds, Ilfracombo rOUp, Morthoe group, Cucull~a-zone, Pilton beds--and says : ' From the Fore- nd on the north to Barnstaple on the south the rocks have an almost uniform dip to the south, usually at a high angle, presenting to all appearance an unbroken succession.' Mr. Etheridge, in his paper of the same year (op. oil. vol. xxiii, p. 568), strongly controverts the views put forward by Prof Jukes, and maintains that the succession in North Devon is one unbroken and continuous series. He subdivides the rocks in ascending order as follows (p. 580) :- Lower j" a. Lynton Sandstone. (Foreland Beds.) Devonian. [ b. Lynton Slates. e. Hangman Grits. Middle. d. Calcareous Slates (fo~siliferous). (Ilfracombe Beds.) Devonian. e. Grey unfossiliferous slates. (Morte Slates.) (jr. Pickwell Down Sandstones. | 9" Baggy and Marwood Slates, etc. ,,I[,%*~P P? r h. Croydon [Croyde] Beds. a)evontan.] i. Braunton Beds. k. Pilton and Barnstaple Beds. He further says at p. 694 : ' After very careful investigation into the physical Btrueture of North Devon, as well as a critical examination of the organic remains contained in its diversified rock-matures, I can come to no other conclusion than that the series of sandstones, slates, and limestones ranging from the Foreland and Lynton on the north to Pilton and Barnstaple on the south are one great and well-defined system, and equally well divisible into three groups, a Lower, Middle, and Upper Devonian series, each equally well characterized by a fauna, the zoological facies of which are sufficiently distinct to determine them one from the other.' r~he following appears to be the final arrangement suggested by Prof.

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