G. R. WATERHOUSE, Curator of the Mu- G. WILLMORE, A.M., Trinity College, Cam seum of the Zoological Society. Zoology (Entomology, Fishes). bridge, Barrister-at-Law. English Law. Rev. Dr. WISEMAN. Catholic Church. W. WITTICH, Teacher of German in Uni versity College, London. Physical Geography Lieut. WOLFE, R.N Rev. S. WOOD. Elocution, Punctuation. R. WORNUM. Organ, Pianoforte. R. N. WORNUM. Lives of Painters, Antient and Modern; Roman, Tuscan, Venetian Schools, &c. G. W. YAPP. Arts and Manufactures." W. YOUATT, author of "The Horse' in the Library of Useful Knowledge. Veterinary Surgery, &c. THE PENNY CYCLOPEDIA OF THE SOCIETY FOR THE DIFFUSION OF USEFUL KNOWLEDGE. WAL WALES, GEOLOGY OF. Though, since the date (1794) of Mr. Aikin's Tour through North Wales,' the geological structure of the principality has been actively and extensively examined by Professor Henslow, Mr. Murchison, Professor Sedgwick, and other eminent persons, there still remain some points unsettled in the general classification of the older Palæozoic strata. These difficulties are now rapidly disappearing under the continued exertions of Professor Sedgwick, Mr. Sharpe, and the zealous members of the Ordnance Geological Survey of Great Britain, directed by Sir H. T. de la Beche. These researches being still in progress, we must wait until the Ordnance surveyors have executed their important task, and solved the grandest and most inviting problem now offered in British geology, by completing a continuous measured section from the Bristol Channel to the Menai Strait, before the whole of the lower strata of Wales can be satisfactorily viewed in one clear and determinate succession of deposits. Still the knowledge we possess of the tendency of these researches is sufficient for a general outline of the mineral composition and organic remains of the whole Paleozoic series of Wales; and it appears desirable to present such an outline, because it must be in some material points different from the opinions which may have been adopted since the Silurian researches of Mr. Murchison and the Cambrian researches of Professor Sedgwick were undertaken, though not to such a degree as to cause to these eminent persons any but gratifying sensations at the progress toward completion of their arduous undertaking. When, in 1831, and many subsequent years, Professor Sedgwick and Mr. Murchison made a friendly partition of labour in Wales, each formed for the country he examined the scheme of classification which seemed most suited to his district. Mr. Murchison, parting from the upper limit of what we have termed the lower Palæozoic series, and working his way downwards through sandy, calcareous, and argillaceous strata (almost unknown, except to Mr. Lewis of Aymestry, and a few intelligent residents in Shropshire, but uncommonly rich in various and successive groups of organic life), established, on a firm basis, the Silurian System; Professor Sedgwick, parting from the lower limit of the same grand series of strata, and proceeding upwards through many thick slates, and conglomerates, and a few thin limestones, in a general sense poor in traces of organic life, proposed to constitute for No. 1. WAL these the Cambrian system. These systems'i ave been adopted into all our geological works with more or less of confident reliance on their being really distinct and recog nisable groups of strata, not merely parts of one grand and varied series of antient deposits. But the conterminous boundary of the groups, the exact line, or even the transition zone between them, was never traced. Mr. Murchison was conducted, by his inquiries downward, into the Cambrian system of Sedgwick, perhaps very deeply into it, but without clearly recognising in the slaty and conglomeritic Cambrians the altered shales and grits of the lower Silumans, and without determining the geographical area of these strata. To determine the geographical extent and geological succession of the Cambrian system was left to Professor Sedgwick, a most arduous and complicated task, the work of many years, and yet unfinished. In this labour he perfectly recognised an important truth, which all subsequent experience confirms, viz. that the remains of organic life in the lowest observed fossiliferous strata of Wales were undistinguishable, except by total number and relative proportion of the several classes of antient life, from the larger series of organic remains in the Silurian strata. That the whole of the lower Palæozoic strata of Wales form in fact one zoological system, was the opinion of Mr. Murchison, expressed in his great work, and from that time a cloud of doubts has gradually deepened over the correctness of the classification which divided this one series of antient life into two systems of stratified depositions. Through this cloud, the only one left on the whole horizon of English stratification, light is breaking by the efforts already alluded to; and we are glad to take this opportunity, the last which may occur, by noticing some points in the geology of Wales, to bring up the knowledge of this subject to the actual date. A short summary of facts will suffice for this end, especially as Mr. Murchison's last address from the Chair of the Geological Society (February, 1843) has touched the same questions. If a line of section be chosen from the shores of the Bristol Channel (as, for example, about Cardiff) across the mountains and valleys to the Menai Strait (a line actually chosen and partly executed by the Ordnance Geological Survey), it may be made to pass through nearly all the principal formations of Wales in a direction favourable for showing the manner of their arrangement. The general features of such a section may be as under : North. Bristel Channel. New Red Sandstone, PIAN STRAT On the shore of the Bristol Channel lias and new redsandstone lie nearly level against and upon the inclined mountain limestone, which supports in a deep basin the coal strata of South Wales. From beneath these, on the north side of the coal-field, rise the old red-sandstone and the Silurian strata, conformable in position to the coal and mountain limestone. I L URIA N 1 North. Cambrian. being a reversal of dip to the north, so that the Silurian strata appear to descend beneath what have been called Cambrian, and have been left under the colour appropriated to the Cambrian rocks in Mr. Murchison's splendid map. These so-called Cambrian strata are however, certainly for many miles northward of the Towy, nothing but the Llandeilo shales, less calcareous and less fossiliferous. The seeming great dip to the north, which often occurs in these beds, is sometimes fallacious, and in fact is caused by the cleavage planes, here generally inclined to the northwards at about 70°. The true beds have however been traced by Sir H. de la Beche and the Ordnance surveyors, and they are found to be at first highly, then moderately inclined to the north, afterRamsay to undulate and roll into anticlinals and synclinals such as constitute the region of the interior of Wales. Section No. 4 may be taken to represent this:-Comparing No. 4 and No. 3, the difference on the north side of the No. 4. The Silurian strata are in their lower parts often confused and somewhat altered by frequent occurrences of trap rocks, and in some places are made to assume a slaty structure, and thus even to lose all distinct stratification. When this happens, the lower limit of the Silurian system appears untraceable; but yet, as a mass, the appearance of these rocks is different from the mixed massive and slaty rocks of the central ranges of Wales, which rise to Plynlymmon, the Berwyns, and Snowdonia. In all of these the stratification is greatly disturbed, often contorted near trap rocks (which are bent with the argillaceous and conglomeritic strata), and generally subject to very prominent slaty cleavage. The least confused part of this labyrinth of rocks is in the Snowdonian range, at least this is the part on which Professor Sedgwick's views appear most positive. Here strata rising to a thickness of many thou-wards to grow flat, and finally have been proved by Mr. sand feet, including slates, conglomerates, and trap bands, succeed one another with considerable regularity, the lowest beds of the series being near the Menai, and there resting upon chloritic and micaceous schists, and meeting unconformed beds of mountain limestone and other newer strata. These beds Professor Sedgwick conceives to be several thousand feet below the limestone of Bala, whose geological relations have been so much discussed. That limestone appeared to himself and Mr. Murchison to dip (eastward) beneath the rocks of the Berwyn mountains, which consist principally of a mass of clay-slate, in which fossils have not yet been discovered. On this point Mr. Sharpe dissents, and gives as the result of his recent examination the Bala limestone lying in a trough between the Berwyns and Arran Fowddy, and resting on the clayslates of the Berwyns. Between the Berwyns and the undoubted Silurian rocks the geographical interval varies. Against the northern parts of the Berwyns the Silurian strata come in contact, but their southern parts are girdled by a broad zone of slates and other rocks, whose age is doubtful; that is to ray, it is not yet determined whether they are of lower Silurian age or of some earlier date. This is not yet determined; but there is information gathered by the Ordnance Geological Survey in the country north of the Towy, which goes far to justify a certain positive inference. The section No. 2 may now be consulted for a general view of the ordinary arrangement of the Silurian strata on the Salopian border of Wales. Here, beneath the coal, mountain-limestone, and old redsandstone, appears the Silurian system, in four parts, resting against the slaty (supposed Cambrian) rocks of the Berwyns. The beds marked 1, Llandeilo flags, are sometimes slaty; 2, the Caradoc sandstones contain conglomerates, and are locally capped by a certain limestone; 3, the Wenlock formation with characteristic limestones; 4, the Ludlow formation, with equally characteristic limestone and peculiar flaggy shales. If this, the normal series of Silurian rocks, retained its characters in all other parts of the border of Wales, nothing could be more easy than to determine the exact geographical range of the system. But this is not the fact. This series exists in perfection only in the eastern and south-eastern parts of the Silurian region, in Shropshire, Woolhope, Malvern, Mayhill, and Usk, and does not exist, with the same parts, in Denbighshire and a great part of South Wales. In the latter district their usual composition may be judged of by section No. 3, where from beneath the old red-sandstone the first Silurian strata which rise to the north are somewhat doubtfully referred to the Ludlow and Wenlock rocks, but are followed by the determinate Caradoc and Llandeilo series. In this series is an anticlinal arch of some considerable length, the effect of it 473? 2 1 a g 1 a North. g section appears great; yet in fact it is merely occasioned by the introduction of the lines of stratification (almost always traceable in almost every rock deposited from water, and under almost every aspect of metamorphism by heat and even fusion), and the separation in the drawing of the arenaceous or conglomeritic beds g, from the shales below (and above, as at s). Now contortions of the stratification similar to those thus traced, in conglomeritic and shaly strata similarly combined, may be traced through a vast breadth of the mountain regions in Wales, which were conceded to the Cambrian system. Slaty cleavage (represented by the fine cross-lines) goes abundantly through these contorted strata, especially through the argillaceous parts, and gives them a general character different from the ordinary aspect of the Llandeilo flags in the Vale of Towy; but this is an effect of particular causes more characteristic of locality than of geological age. From these facts a general presumption arises that the slaty rocks in the interior of Wales may not be really of higher antiquity than the Llandeilo shales and grits. What, upon this supposition, is the Bala limestone? What are the Snowdonian slates and conglomerates? Complete answers to these questions cannot be now given; nor will they be completely answered till the measured work of the Ordnance Geological Survey has been carried across the whole of Wales in the direction already indicated. But answers have been attempted, partly on the evidence of sections, partly on the evidence of organic remains. On the evidence of accurate (but limited) sections, Mr. Sharpe shows that the Bala limestone presents much conformity with the limestone of Coniston in Cumberland, now admitted to be of Lower Silurian age; and that, in common with other associated beds, it contains the fossils of that age, was well known to Professor Sedgwick and Mr. Murchison. But Mr. Sharpe has added the statement, that these Bala beds, folding over to the east and south, are surmounted by Upper Silurian beds.' (Murchison, in the Address already referred to, p. 14.) If this conclusion have even only a local value, it is, taken in connection with the proved undulations of the Lower Silurian beds north of the Towy, of great importance, and, supposing it established eventually on a greater scale, we shall find that but a small portion of any strata older than the Lower Silurians can be reasonably looked for in the central regions of Wales. Perhaps the lowest clay-slates of the Berwyng may be in this case. Admitting, with Professor Sedgwick, that the fossiliferous Snowdonian slates and conglomerates are really placed several thousand feet below the Bala limestone, and are parts of a series extending several thousand feet still |