On the Physical Structure of North Devon, and on the Palæontological Value of the Devonian Fossils

Abstract

Abstract T he Lower, Middle, and Upper groups of sandstones and shales of West Somerset and North Devon are described in this paper as occurring in a regular and unbroken succession from north to south—namely, from the sandstones comprising the promontory of the Foreland, at the base, to the grits and slates &c. overlying the Upper Old Red Sandstone of Pickwell Down to the south. The author has been unable to see any trace of a fault of sufficient magnitude to invert the order of succession, or that would cause the rocks of the Foreland at Lynton to be upon the same horizon as those south of a line of high ground that passes across the county from Morte Bay on the west to Wiveliscombe on the east. The Foreland grits and sandstones are overlain by the Lower or Lynton slates, and form a group equal in time to the Lower Old Red Sandstone of other districts, but deposited under purely marine conditions. The author then shows that above the Lower or Lynton slates there is an extensively developed series of red, claret-coloured, and grey grits, from 1530 to 1800 feet thick; these form a natural and conformable base to the Middle Devonian or Ilfracombe group. The highest beds, containing Myalina and Natica , insensibly pass into the gritty and calcareous slates of Combe Martin, Ilfracombe, &c.; this Middle group Mr. Etheridge unhesitatingly regards as the equivalent of the Torquay and Newton Bushel series of South Devon. Mr. Etheridge gives detailed Tables of the organic remains of the two groups (the Lower, or Lynton, and the Middle or Ilfracombe), and collates with them those species found in equivalent strata in Rhenish Prussia, Belgium, and France. He is inclined to believe that these two marine fossiliferous groups represent in time the unfossiliferous Old Red Sandstone (Dingle beds) of Kerry, and the Glengariff and Killarney Grits of the south-west of Ireland. The author then endeavours to prove that the Pickwell Down beds are the true Upper Old Red Sandstone only, not the whole of the formation, as was lately proposed. Arguments are also brought forward to show the probability of the Carboniferous slate (in part) and Coomhola grits being the equivalent of the English Upper Old Red Sandstone, or Upper Devonian, and that the North Devon beds only are to be regarded as the true type, to which the Irish must be compared, and not vice versâ . Physical and palæontological evidence distinctly proves, the author states, that the whole of the slates and limestones of Lee, Ilfracombe, and Combe Martin underlie the Morte Bay Red Sandstones. The author compares the whole of the Devonian fauna of Britain with that of the Rhine, Belgium, and France, by means of series of Tables based upon the British types. These marine Devonian species are compared with those of the Old Red Sandstone proper, the Silurian, and the Carboniferous; and analyses are made of all the classes, orders, genera, and species, in relation to the groups of rocks in which they occur,—the result being the conclusion that the marine Devonian series, as a whole, constitutes an important and definite system.