The Story of the Earth and Man

by Sir John William Dawson

THE LOWER AND UPPER SILURIAN AGES.

By English geologists, the great series of formations which succeeds to the Cambrian is usually included under the name Silurian System, first proposed by Sir Roderick Murchison. It certainly, however, consists of two distinct groups, holding the second and third faunas of Barrande. The older of the two, usually called the Lower Silurian, is the Upper Cambrian of Sedgwick, and may properly be called the Siluro-Cambrian. The newer is the true Silurian, or Silurian proper—the Upper Silurian of Murchison. We shall in this chapter, for convenience, consider both in connection, using occasionally the term Lower Silurian as equivalent to Siluro-Cambrian. The Silurian presents us with a definite physical geography, for the northern hemisphere at least; and this physical geography is a key to the life conditions of the time. The North American continent, from its great unbroken area, affords, as usual, the best means of appreciating this. In this period the northern currents, acting perhaps in harmony with old Laurentian outcrops, had deposited in the sea two long submarine ridges, running to the southward from the extreme ends of the Laurentian nucleus, and constituting the foundations of the present ridges of the Roeky Mountains and the Alleghanies. Between these the extensive triangular area now constituting the greater part of North America, was a shallow oceanic plateau, sheltered from the cold polar currents by the Laurentian land on the north, and separated by the ridges already mentioned from the Atlantic and Pacific. It was on this great plateau of warm and sheltered ocean that what we call the Silurian fauna lived; while of the creatures that inhabited the depths of the great bounding oceans, whose abysses must have been far deeper and at a much lower temperature, we know little. During the long Silurian periods, it is true, the great American plateau underwent many revolutions, sometimes being more deeply submerged, and having clear water tenanted by vast numbers of corals and shell-fishes, at others rising so as to become shallow and to receive deposits of sand and mud; but it was always distinct from the oceanic area without. In Europe, in like manner, there seems to have been a great internal plateau bounded by the embryo hills of Western Europe on the west, and harbouring a very similar assemblage of creatures to those existing in America.

Further, during these long periods there were great changes, from a fauna of somewhat primordial type up to a new order of things in the Upper Silurian, tending toward the novelties which were introduced in the succeeding Devonian and Carboniferous. We may, in the first place, sketch these changes as they occurred on the two great continental plateaus, noting as we proceed sucli hints as can be obtained with reference to the more extensive oceanic spaces.

Before the beginning of the age, both plateaus seem to have been invaded by sandy and muddy sediments charged at some periods and places with magnesian limestone; and these circumstances were not favourable to the existence or preservation of organic remains. Such are the Potsdam and Calciferous beds of America and the Tremadoc and Llandeilo beds of England. The Potsdam and Tremadoc are by their fossils included in the Cambrian, and may at least be regarded as transition groups. It is further to be observed, in the case of these beds, that if we begin at the west side of Europe and proceed easterly, or at the east side of America and proceed westerly, they become progressively thinner, the greater amount of material being deposited at the edges of the future continents; just as on the sides of a muddy tideway the flats are higher, and the more coarse sediment deposited near the margin of the channel, and fine mud is deposited at a greater distance and in thinner beds. The cause, however, on the great scale of the Atlantic, was somewhat different, ancient ridges determining the border of the channel. This statement holds good not only of these older beds, but of the whole of the Silurian, and of the succeeding Devonian and Carboniferous, all deposited on these same plateaus. Thus, in the case of the Silurian in England and Wales, the whole series is more than 20,000 feet thick, but in Russia, it is less than 1,000 feet. In the eastern part of America the thickness is estimated at quite as great an amount as in Europe, while in the region of the Mississippi the Silurian rocks are scarcely thicker than in Russia, and consist in great part of limestones and fine sediments, the sandstones and conglomerates thinning out rapidly eastward of the Appalachian Mountains.

In both plateaus the earlier period of coarse accumulations was succeeded by one in which was clear water depositing little earthy sediment, and this usually fine; and in which the sea swarmed with animal life, from the débris of which enormous beds of limestone were formed the Trenton limestone of America and the Bala limestone of Europe. The fossils of this part of the series open up to us the head-quarters of Lower Silurian life, the second great fauna of Barrande, that of the Upper Cambrian of Sedgwick; and in America more especially, the Trenton and its associated limestones can be traced over forty degrees of longitude; and throughout the whole of this space its principal beds are composed entirely of comminuted corals, shells, and crinoids, and studded with organisms of the same kinds still retaining their forms. Out of these seas, in the European area, arose in places volcanic islets, like those of the modern Pacific.

In the next succeeding era the clear waters became again invaded with muddy and sandy sediments, in various alternations, and with occasional bands of limestone, constituting the Caradoc beds of Britain and the Utica and Hudson River groups of America. During the deposition of these, the abounding life of the Siluro-Cambrian plateaus died away, and a middle group of sandstones and shales, the Oneida and Medina of America and the Mayhill of England, form the base of the Upper Silurian.

But what was taking place meanwhile in the oceanic areas separating our plateaus? These were identical with the basins of the Atlantic and Pacific, which already existed in this period as depressions of the earth’s crust, perhaps not so deep as at present. As to the deposits in their deeper portions we know nothing; but on the margin of the Atlantic area are some rocks which give us at least a little information.

In the later part of the Cambrian period the enormous thickness of the Quebec group of North America appears to represent a broad stripe of deep water parallel to the eastern edge of the American plateau, and in which an immense thickness of beds of sand and mud was deposited with very few fossils, except in particular beds, and these of a more primordial aspect than those of the plateau itself. These rocks no doubt represent the margin of a deep Atlantic area, over which cold currents destructive of life were constantly passing, and in which great quantities of sand and mud, swept from the icy regions of the North, were continually being laid. The researches of Dr. Carpenter and Dr. Wyville Thomson show us that there are at present cold areas in the deeper parts of the Atlantic, on the European side, as we have long known that they exist at less depths on the American side; and these same researches, with the soundings on the American banks, show that sand and gravel may be deposited not merely on shallows, but in the depths of the ocean, provided that these depths are pervaded by cold and heavy currents capable of eroding the bottom, and of moving coarse material. The Quebec group in Canada and the United States, and the metalliferous Lower Silurian rocks of Nova Scotia and Newfoundland, destitute of great marine limestones and coral reefs, evidently represent deep and cold-water areas on the border of the Atlantic plateau.

At a later period, the beginning of the Upper Silurian, the richly fossiliferous and exceptional deposits of the Island of Anticosti, formed in the deep hollow of the Gulf of St. Laurence, show that when the plateau had become shallowed up by deposition and elevation, and converted into desolate sand-banks, the area of abundant life was transferred to the still deep Atlantic basin and its bordering bays, in which the forms of Lower Silurian life continued to exist until they were mixed up with those of the Upper Silurian.

If we turn now to these latter rocks, and inquire as to their conditions on our two great plateaus, we shall find a repetition of changes similar to those which occurred in the times preceding. The sandy shallows of the earlier part of this period give place to wide oceanic areas similar to those of the Lower Silurian; In these we find vast and thick coral and shell limestones, the Wenlock of England and Niagara of America, as rich in life as the limestones of the Lower Silurian, and with the generic and family forms similar, but the species for the most part different. In America these limestones were followed by a singularly shallow condition of the plateau, in which the surface was so raised as at times to be converted into separate salt lakes in which beds of salt were deposited. On both plateaus there were alternations of oceanic and shallow conditions, under which the Lower Helderberg and Ludlow beds, the closing members of the Silurian, were laid down. Of the Atlantic beds of this period we know little, except that the great limestones appear to be wanting, and to be replaced by sandy and muddy deposits, in some parts at least of the margins of the area. In some portions also of the plateaus and their margins, extensive volcanic outbursts seem to have occurred; so that the American plateau presented, at least in parts, the aspect of a coral sea with archipelagos of volcanic islands, the ejections from which became mixed with the aqueous deposits forming around them.