Silurian

“ Mineral Character of Silurian Strata.
In lithological character, the Silurian strata vary greatly when we trace them through Europe and North America. The shales called mudstones are as little altered from some deposits, found in recent submarine banks, as are those of many tertiary formations. We meet with red sandstone and red marl, with gypsum and salt, of Upper Silurian date, in the Niagara district, which might be mistaken for trias. The whitish granular sandstone at the base of the [p.361] Silurian series in Sweden resembles the tertiary siliceous grit of Fontainebleau. ”

“ Unlike the Ordovician period, the Silurian closed without any mountain-making disturbance or great uplift of land. The Silurian period, like the preceding Ordovician and Cambrian, seems to have been free from any more than slight igneous activity as, for example, in Maine and New Brunswick. The total thickness of Silurian strata in North America is seldom more than a few thousand feet. ”

“ Were the trilobites of the Silurian system,—at one period, as their remains testify, more than equally abundant,—creatures of similar habits? We have at length arrived at the tall sandstone precipices of Dunnet, with their broad decaying fronts of red and yellow; but in vain may we ply hammer and chisel among them: not a scale, not a plate, not even the stain of an imperfect fucoid appears. We have reached the upper boundary of the Lower Old Red formation, and find it bordered by a desert devoid of all trace of life. ”

“ I therefore prefer to regard them as the base of the Upper Silurian, to which group they are linked by more than twice as many species as to the Lower Silurian. By this arrangement the line of demarkation between the two great divisions, though confessedly arbitrary, is less so than by any other. They are called Llandovery Rocks, from a town in South Wales, in the neighbourhood of which they are well developed, and where, especially at a hill called Noeth Grug, in spite of several faults, their relations to one another can be clearly seen. ”

“ For example, during the Silurian period all the earth was Silurian, and animals were living and dying, and depositing their remains more or less over the whole area of the globe, and they were probably (the species at least) nearly as varied in different latitudes and longitudes as at present. What proportion do the Silurian districts bear to the whole surface of the globe, land and sea (for far more extensive Silurian districts probably exist beneath the ocean than above it) , and what portion of the known Silurian districts has been actually examined for fossils? ”

“ The Silurian beds are almost every where greatly bent, compressed, and dislocated; the strike is in the great majority of cases from south-west to north east. At and around Christiania, in the tracts bordering on Lake Mjösen and Skien Fjord, the Silurian beds occur without being metamorphosed, except locally at their contact with eruptive rocks. In the environs of Bergen, the outer part of Hardanger Fjord, the Hardanger Waste, in Sondre Throndhjem amt we meet with regionally metamorphic schists and limestones containing Silurian fossils. ”

“ But the descriptions which we possess of the Silurian deposits over immense territories in Russia and in North America, do not support the view that the older a formation is the more invariably it has suffered extreme denudation and metamorphism. ”

“ Again, going farther back, we find the lapse of time represented by the Silurian period to be even more striking than that of the Old Red Sandstone. The unconformities in the Silurian series indicate that many thousands of feet of these strata were denuded before overlying members of the same great formations were deposited. And again, this immense formation was formed in the ocean by the slow denudation of pre-existing Cambrian continents, just as these had been built up out of the ruins of the still prior Laurentian land. ”

“ The result was the establishment of the Silurian system—under which were grouped for the first time a remarkable series of formations, each replete with distinctive organic remains older than and very different from those of the other rocks of England. ”

“ Silurian.—Upper Silurian, having a strong relation to the Wenlock group of Britain, but with an American facies, and Lower Silurian, with a succession much the same as in British North America, are found on the shores of Smith Sound, and Nathorst has discovered them in King Oscar Fjord, but not as yet so far south as the Danish possessions.
3. Devonian rocks are believed to occur in Igaliko and Tunnudiorbik Fjords, in S.W. Greenland, but as they are unfossiliferous sandstone, rapidly disintegrating, this cannot be known. ”

“ Doubtless the fact that the Carboniferous forms so greatly outnumber the Silurian has a meaning. Does not this greater organic affinity betoken a closer connection with the more modern than with the more ancient period? ”

“ In several parts of the world the Hypozoic Strata of gneiss and mica schist are succeeded immediately, or almost immediately, by the Silurian strata, as they were defined in 1836 by sir R. Murchison (Scandinavia, Bohemia, &c.) , but in others, (Wales, Cumberland) the Llandeilo rocks, which were assumed as the original base of the Silurians, rest upon a great thickness of arenaceous and argillaceous deposits, among which clay slate is very abundant, and in which organic life is rare. Professor Sedgwick regarded these from 1832 to 1836 as the Cumbrian and lower part of his Cambrian systems ”

“ In reference to the great geological truth that the earlier formations, stretching over vast extent, and perfectly unaltered, do not contain the least traces of organized beings, and that the dawn of life is in the lower Silurian formation, our author admits that "if his theory be true, it is indisputable that before the lowest Silurian stratum was deposited long periods elapsed, as long as, or probably far longer, than the whole interval from the Silurian age to the present day, and that during these vast yet quite unknown periods of time the world swarmed with living creatures." ”

“ For instance, I cannot doubt that all the Silurian trilobites have descended from some one crustacean, which must have lived long before the Silurian age, and which probably differed greatly from any known animal. Some of the most ancient Silurian animals, as the Nautilus, Lingula, &c., do not differ much from living species; and it cannot on my theory be supposed, that these old species were the progenitors of all the species of the orders to which they belong, for they do not present characters in any degree intermediate between them. ”

“ Bearing in mind, however, that the Silurian is as remote from the Laurentian on the one hand as from the Tertiary on the other, we might be prepared to expect that if the Layer-corals of the Silurian are divisible into different groups, somewhat widely separated, and we have in the lower Palæozoic the peculiar type of Cryptozoon, we may be prepared to expect in the Laurentian much more generalized forms, less susceptible of classification in our modern systems. ”

“ In the Silurian districts no such terraces or escarpments exist, the general high dip of the strata, which often approaches the vertical, precluding any such contour. In a region composed of highly-inclined greywacké and shale, however, we should expect to find that where the strata are of unequal durability, the harder beds will stand up in long narrow ridges, separated by intervening hollows, which — 99 — have been worn out along the outcrops of the softer and more easily-denuded beds. And such appearances do show themselves in some parts of the Silurian area. ”

“ Wherever the fluids of the living body can penetrate, there also mineral substances can be carried, and this natural injection, effected under great pressure and with the advantage of ample time, can surpass any of the feats of the anatomical manipulator. Fig. 25 represents a microscopic joint of a Crinoid from the Upper Silurian of New Brunswick, injected with the hydrous silicate already referred to, and fig. 26 shows a microscopic « 105 » chambered or spiral shell, from a Welsh Silurian limestone, with its cavities filled with a similar substance. ”

“ In Norway, in Germany, in France, in Russia, in Siberia, in Kamtchatka, in parts of South America, in short, wherever the civilization of the white race has extended, Silurian deposits have been observed, and everywhere they bear the same testimony to a profuse and varied creation. The earth was teeming then with life as now, and in whatever corner of its surface the geologist finds the old strata, they hold a dead fauna as numerous as that which lives and moves above it. ”

“ Silurian " was first introduced by Sir R. I. Murchison in 1835 for a series of rocks on the border counties of England and Wales—a region formerly inhabited by the Silures. Murchison's Silurian embraced not only the rock groups indicated above, but others below them that were much older, even such as are now classed as Cambrian. About the same time A. Sedgwick proposed the term Cambrian for a great succession of rocks which includes much of Murchison's Silurian system in its upper part; hence arose that controversy which left so lasting a mark on British geology. ”

“ This divergence is the most marked in the Palæozoic forms of life, less so in those of the Mesozoic period, and less still in the Tertiary period. Each successive formation has therefore presented us with animals becoming gradually more and more like those now in existence; and though there is an immense and striking difference between the Silurian animals and those of to-day, this difference is greatly reduced if we compare the Silurian fauna with the Devonian ”

“ The immense accumulations of Silurian sediment had by this time so overweighted certain portions of the crust, that great quantities of aqueous sediment had been pressed downward into the heated bowels of the earth, and were undergoing, under an enormous weight of superincumbent material, a process of baking and semi-fusion. This process was of course extremely active along the margins of the old Silurian plateaus, and led to great elevation of land, while in the more central parts of the plateaus the oceanic conditions still continued ”

“ In parts of the globe still more remote from Europe the Silurian strata have also been recognised, as in South America, Australia, and India. In all these regions the facies of the fauna, or the types of organic life, enable us to recognise the contemporaneous origin of the rocks; but the fossil species are distinct, showing that the old notion of a universal diffusion throughout the ”primæval seas” of one uniform specific fauna was quite unfounded, geographical provinces having evidently existed in the oldest as in the most modern times. ”

“ In Goeppert’s memoir on the flora of the Silurian, Devonian, and Lower Carboniferous rocks, published in 1860, [AI] he enumerates twenty species as Silurian, but these are all admitted to be Algæ, and several of them are remains which may be fairly claimed by the zoologists as zoophytes, or trails of worms and mollusks. ”

“ The Silurian rocks, lower and upper, which in these islands have their chief development in Wales, and which are nearly thirty-eight thousand feet thick; and the Devonian or Old Red sandstone beds, which in the Fans of Brecon and Carmarthenshire attain a thickness of ten thousand feet, must be passed through in an upward direction before we reach the bottom of that Carboniferous Limestone of which I spoke in my last paper. ”

“ The Silurian rocks in all countries are very poor in foraminiferal shells, only occasional examples being found. In rocks of this age at Lilydale, Victoria, the genus Ammodiscus, with fine sandy, coiled tests, is found in the Cave Hill Limestone. ”

“ In support of this view, I have elsewhere referred to the deposition of silicates of lime, magnesia, and iron from natural waters, to the great beds of sepiolite in the unaltered Tertiary strata of Europe; to the contemporaneous formation of neolite (an aluimino-magnesian silicate related to loganite and chlorite in composition) ; and to glauconite, which occurs not only in Secondary, Tertiary, and Recent deposits, but also, as I have shown, in « 119 » Lower Silurian strata. ”

“ But, just because of the extraordinary fundamental importance of Eozoön, and because the discovery of that fossil adds several millions of years to the earth's organic history, making the primitive Silurian formations to appear recent by comparison, and rendering a great service to the doctrine of evolution, therefore it is that the opponents of that doctrine so stoutly affirm that it is not of organic origin at all, but purely mineral. ”

“ Much of the land area then sank below sea level, and became covered with sand, gravel and the like. The portion which remained above the level is called the Archaean. Later a general elevation of the land area brought above sea level much of this land and gravel, forming around the Archaean an increased land era, which we call Silurian. The time when the sand and gravel was deposited forming this land is known as the Silurian age. Following this came the Devonian age. ”

“ In coming to America it was my wish to become more intimately acquainted with the different Silurian stages, and especially with those adjacent to the Waterlime group, i.e., with the Onondaga and Guelph limestones on the one side and the Tentaculite limestone on the other. It would perhaps be possible to find other connecting links in the development of life in both countries. ”