VII. On the Physical Structure of those Formations which are immediately associated with the Primitive Ridge of DEVONSHIRE and CORNWAll. BY THE REV. ADAM SEDGWICK, M.A. M.G.S. FELLOW OF TRINITY COLLEGE, AND WOODWARDIAN PROFESSOR IN THE UNIVERSITY OF CAMBRIDGE; SECRETARY TO THE CAMBRIDGE PHILOSOPHICAL SOCIETY. [Read March 20, 1820.] CONTENTS. §. 1. §. 2. §. 3. Red Sandstone, Conglomerate, and Greywacké of Somersetshire. §. 1. Red Sandstone, Conglomerate, and Greywacké of Somersetshire, &c. THE observations which I have the honour to present to the Cambridge Philosophical Society, are principally compiled from memoranda made in the summer of the year 1819, during a tour in Cornwall and some of the western parts of Devonshire. In examining the most interesting portions of that country, I was accompanied by the Rev. W. R. Gilby, Fellow of Trinity College, to whose active assistance I am happy in having it in my power thus publicly to express my obligation. M It may perhaps appear useless to bring together only such notices of this part of our Island as can be made during a hasty tour. But no regular survey has yet been made of this district by persons qualified at the same time to explain its general structure, and to exhibit the phenomena presented by its more minute features. Such a survey we may indeed expect from the combined labours of the Cornish Geological Society. Until, however, it is completed and given to the public, notices like the present may not be without their advantage. As an additional reason for submitting this paper to the University, I may observe, that during the last year, several hundred specimens have been placed in the cabinets of the Woodwardian Museum; many of them selected from the tract now under consideration. But as a mere catalogue, stating the locality and mineral character of the several specimens, can by no means give all the information they are intended to convey; it is proposed, by this Memoir, to confer an additional value on a portion of our collection, by explaining the views under which it was selected and arranged. Before entering on the primitive region, which it is the more immediate object of this paper to describe, I devoted some time to an examination of the sandstone and conglomerate which are so widely diffused among the schistose rocks of Somersetshire. Many portions of that deposit are described with great fidelity by Mr. Horner in the Geological Transactions*. I therefore refer my hearers to his Memoir, and to the specimens from the same deposit, which are placed in the Woodwardian Museum, and accompanied by a descriptive catalogue. Similar deposits may be traced in almost uninterrupted succession to the base of the Mendip Hills; and still farther north they are seen resting in the same unconformable position on the coal strata in the neighbonrhood of Bristol. In the Geological Transactions, vol. III. p. 342, &c. vallies of the eastern parts of Devonshire they occupy a wide extent, and, as far as I observed them, appear always to bear the same relation to the rocks with which they are associated. The quarries in the neighbourhood of Heavitree and Upton Pyne, and the red sandstone of the valley of the Ex, have the undoubted characters of belonging to the same epoch. On the whole then, it appears, that the red sandstone and conglomerate of Somersetshire and Devonshire are members of the new red sandstone formation, which is so widely extended in many parts of this Island. It is not intended to deny, that a sandstone, coeval with that which crops out from beneath the mountain limestone on the banks of the Avon, may be found among the schistose rocks of Devonshire. What has been said refers only to those parts examined by myself, and from which the specimens in our Museum have been selected *. The slaty rocks associated with the red sandstone and conglomerates before mentioned, first make their appearance on the west side of the great alluvial plain of Bridgewater. They soon rise out in gentle elevations, and may be traced, almost without interruption, through the commanding ridge of the Quantock Hills. To the west of this ridge, mountain masses composed of the same rock extend in uninterrupted succession into the northern and eastern parts of Devonshire. This tract of country, almost throughout its whole extent, is singularly diversified; presenting a rapid succession of those undulations which are so characteristic of this beautiful portion of our Island. Diluvian action, and the unequal decomposition of the mineral masses which form the subsoil, have been mentioned among the causes of these singular inequalities. It * These observations were made in consequence of a mistatement which the Author of this paper had remarked in the best Geological authorities with which he was at that time acquainted. He is happy to find that his own views have been confirmed in the admirable Geological Map of England published by Mr. Greenough. is however to the constitution of the rocks themselves that we are principally to look for the explanation of this appearance. Throughout the greater part of their extent they exhibit most extraordinary contortions. In many parts of the coast of Somerset and North Devon, where we have the finest natural sections of these schistose masses, the strata do not preserve the same direction for many feet together; but are continually deflected, and coiled on each other; presenting in their course lines of every possible curvature. The same vertical section often exposes a series of beds, one portion of which is disposed in curves convex, and another portion in curves concave to the horizon; and so situated in respect of each other, that no force whatever, acting in a given direction, could produce the structure out of masses originally in an horizontal position. When the fundamental rock is thus constituted, the country must present a corresponding variety of surface. The whole formation has generally been classed with the greywacké rocks of the German school. Many beds are of silky lustre, and even when viewed with a lens, appear of homogeneous texture. But in the cliffs, and other places where the formation is best exposed, we find the finest grained beds alternating, often without any apparent order, with rocks which exactly agree with the definition of greywacké. The term itself, therefore, as long as it shall be retained in the barbarous nomenclature of Geology, may be applied to this formation with sufficient propriety. It is not my intention to enter into any details respecting this formation; I therefore refer to the Geological Transactions*, and only add, that among the characteristic specimens, placed in the Woodwardian Museum, will be found the following varieties† : 1. Quartz rock. 2. Silicious sandstone, coarse-grained. 3. The same coarse-grained silicious sandstone containing splinters of slate. * Vol. II. p. 495, and vol. III. p. 338. + See the specimens arranged under the Greywacké of Somersetshire. 4. The same substances with the trace of an argillaceous cement. 5. A rock, where the base is distinctly argillaceous with imbedded pieces of quartz and splinters of slate*. 6. The same compound but much finer grained, of slaty texture and generally micaceous. 7. A schistose amygdaloid; the base a variety of clay-slate. 8. The fine homogeneous slate; surface silky and of various cloudy colours; in some instances micaceous, &c. The characters above-mentioned would by some be considered at once sufficient to determine the place of these rocks in the transition class. The absence of all organic remains from, at least, the greatest part of them, and the uncertainty of classifications deduced from the mere consideration of mineral character, might throw some doubt on this conclusion, if we were not assisted in our determination by many calcareous masses, which are irregularly imbedded in the greywacké, and which constantly present most indisputable traces of organic remains. Several specimens have been selected from these calcareous beds; and from their external characters and geological situation, as well as from a consideration of their organic remains; I think that they may be referred to a formation distinct from the mountain limestone, and belonging to an earlier epoch. The existence of organic remains in calcareous beds, which alternate with rocks where some of the other earths predominate, and in which we trace no vestige of previous animal existence, is a phenomenon by no means unusual, even in much more recent formations. Water contaminated by a large portion of silex, either in a state of mechanical mixture or chemical solution, was probably unfit for the support of animal life. We must recollect too, that calcareous matter was necessary to the very existence of these inhabitants of the ancient bed of the ocean. They could therefore be abundantly propagated, only where the sea held a sufficient quantity of calcarious matter in solution. * These specimens, were it not for their intimate association with the finer beds might be easily mistaken for varieties of old red sandstone. |