anticline, though very quartzose, are partially constructed of felsite grains." If we next consider the Upper Cambrian we find they also are thickest in Merioneth, being there no less than 7,000 feet thick. Near Llanberis, west of Snowdon, the Lingula Flags are only 2,300 feet, and there is little that can be called Tremadoc. Finally, near Bangor, the Lingula Flags are represented by sandstones and mudstones resting on the grits which overlie the basal conglomerate; even if the basement beds are also part of the same series (see ante, p. 17) the total thickness is not more than 1,000 feet, and there is no representative of the Tremadoc. A similar change takes place southward; near St. Davids the Lingula Flags are not more than 2,000 feet thick, and consist largely of sandstones and flags, many of the beds showing ripple marks, and all attesting shore conditions. The Tremadoc slates are 1,000 feet thick. At Malvern there is 500 feet of sandstone resting on Archæan, and overlain by 1,000 feet of shale belonging to the Dolgelly and Tremadoc groups, and in Shropshire there is a similar sequence. In Warwickshire, however, the series again thickens; there is a massive quartzite of uncertain age, but probably Upper Cambrian, overlain by 2,000 feet, of Upper Cambrian shale. The Cambrian rocks of Wales seem, therefore, to have been formed in a deep trough between lofty ridges formed of the Archæan rocks on the east and on the west, and possibly also to the south-east. The only Irish rocks referable to the Cambrian system are those in Wicklow and Wexford.1 Lithologically they are comparable to the Lower Cambrian of Wales and Shropshire, but as their base is not seen, we do not know on what they rest, and no definite fossils have yet been found in them. 1 The Galway rocks, regarded as Cambrian by Mr. Kinahan, are more probably of Archæan age. In Scotland we once more find rocks resting on undoubted Archæan and covered by Ordovician, but they are very different lithologically from the Cambrian of Wales. They consist entirely of red felspathic sandstones, with beds of breccia and conglomerate at the base, and are in places from 3,000 to 4,000 feet thick. This Torridon Sandstone is probably of early Cambrian age, and has evidently been formed against an Archæan coast-line. Its composition is thus described by Professor Bonney 1: "Its coarser basement beds are crowded with fragments of the underlying gneisses and schists, and since the epoch of their formation no important change has taken place in either the one or the other. The finer beds, though other materials occasionally occur, are largely, sometimes almost exclusively, composed of grains of quartz and of felspar identical in every respect with those of the underlying series. It may be a fact of some significance, for it agrees with what I have elsewhere noticed in very old fragmental rocks, that the felspar appears to have been broken off from the parent rock while still undecomposed, and in many cases is even now remarkably well preserved. It would seem, therefore, as if the denudation of the granitoid rock had been accomplished without material decomposition of its felspar; but I must not allow myself to digress into speculations on this interesting and suggestive fact." The Torridon Sandstone has not, however, been entirely derived from the Archæan gneiss. Mr. B. N. Peach states that in some parts of Sutherland the basal conglomerate consists chiefly of stones derived from older sedimentary rocks, such as greywacke, quartzite, hardened shales, and cherty limestones, together with a few pebbles of slaggy diabase lava. Professor Judd remarks 2: "These rocks in their cha 1 Pres. Address to Geol. Sec. of Brit. Assoc., 1886. racters and their relations so greatly resemble the Sparagmite Formation of Scandinavia, that it is impossible to refrain from drawing comparisons between them. The Scandinavian formation, however, includes calcareous and slaty deposits, which are wanting in its Scottish analogue;" and he points out that as Upper Cambrian fossils have been found at the very top of the Sparagmite series, this fact lends support to the view that the Torridon Sandstones are of Lower Cambrian age. § 2. Geographical Restoration. The relations which the Cambrian sediments bear to the underlying Archæan, both in this country and elsewhere, are so remarkable that we are driven to conclude that the physical conditions of the earth's surface at this early period of its history were very different from that which it subsequently acquired. At the commencement of what we call the Cambrian period, the surface of the earth seems to have been extraordinarily rugged and uneven, exhibiting a series of lofty mountain ridges separated by deep troughs and hollows, the bottoms of which were 10,000 or 12,000 feet below the summits of the ridges. How such inequalities were formed is a difficult question to answer. Were they the result of the mode in which the Archæan rocks were originally formed, or were they partly the result of rain and rivers acting on a newly-formed crust? The conglomerates and coarse sandstones which are so frequent in the Lower Cambrian series, and the unweathered state of the felspar grains in the grits, are suggestive facts, and one cannot help thinking that these rocks must have been accumulated with much greater rapidity than could be effected by any modern process of formation. An obvious speculation suggests itself: Are we looking upon the result of the first condensation of water upon the earth's surface, and was the Cambrian sea the first great body of water that ever lay over the European area? It must be admitted that there are some grounds for so thinking; there may have been lower levels and older oceans in other parts of the world, but so far as we know the Cambrian rocks are the oldest aqueous deposits in Europe, and the Caerfai beds of South Wales contain the oldest remains of invertebrate animals; moreover, though these animals are certainly the ancestors of modern marine species, the waters of the Cambrian sea may not have been salt; and we have no proof that these creatures had been differentiated into salt- and fresh-water forms until we reach the close of the Devonian period. Dr. Hicks has discussed the general geographical conditions of the Cambrian period,' and he thinks that the higher parts of the pre-Cambrian land lay toward the north-east of Europe, and that the surface had a general slope from north-east to south-west, but was traversed by mountain ridges having a general E.N.E. and W. S. W. direction. If this were so, and subsidence took place, the part of the surface which faced south-west would first become covered by the sea. He infers from a consideration of the Cambrian series in various parts of Europe that the sea gradually spread further and further to the north-east, and that there was a difference in level of 15,000 feet between the low ground of the south-west and the high ground in Russia. The late Professor Linnarsson, however, entirely differed from Dr. Hicks. He points out that the Scandinavian succession goes nearly, if not quite, as low as that of South Wales, and further that, though the sandstones below the Scandinavian Menevian are not so thick as the British, this 1 "Quart. Journ. Geol. Soc.," vol. xxxi., p. 552. 2" Geol. Mag.," 1876, p. 145. may have been mainly due to a less rapid rate of deposition. "If Mr. Hicks' views of the physical geography of the Cambrian period were correct, there ought to be in the middle and upper portions of the Swedish series many signs of littoral conditions. . . . The facts, however, rather tend to show that most of the Swedish Cambrian rocks were deposited in a deeper sea and farther from land than the British." The fact is that in Scandinavia, as in Britain, there are great variations in the thickness of the Lower Cambrian rocks; thus in Norway the lowermost sandstones are 2,000 feet thick, while in Sweden they are only 100 feet. Again, the Olenus beds in Norway contain sandstones and quartzites, but in Sweden they are wholly shales with bands of limestone. These facts point rather to the conclusion that the greatest mass of land lay to the north-west, for coarse sediments are likely to thicken in the direction of land and not away from it, as Dr. Hicks seems to suppose. The Russian succession is not antagonistic to this conclusion, for the whole Cambrian series is not 700 feet thick; the basal sandstones are 300 feet, and the overlying beds are such as would be formed at a distance from land, being blue clays surmounted by glauconitic shales and limestones like some of our Cretaceous beds. All the available evidence certainly seems to favour the view taken by Professor Hull,' that the greater mass of dry land lay to the north-west of Europe, and occupied a large part of what is now the North Atlantic Ocean. There may have been smaller but still extensive tracts of land of the European area, as is plainly indicated in England and Bohemia; nay, it would appear that in Lower Cambrian times there was in Europe more land than sea, and that the subsequent changes throughout the Upper Cambrian and Ordovician periods tended to reverse this state of things Physical History of the British Isles," p. 61. 1 66 |