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after the chalk formation, and they increase in number gradually during the Tertiary period.1

2

It is no doubt unsafe to draw conclusions as to the nature of the fauna and flora of the separate periods from the fossils which are found in the separate strata; for, on the one hand, as I have already observed, those fossils which have been preserved are not yet thoroughly known; and, on the other hand, many organisms which may have existed were either not of a kind to be pétrified,—as fungi, slugs, etc., which are entirely wanting in hardness,—or could not be petrified because they existed on land, on high mountains, or in the air. Many geologists have not noticed these facts, and have hastily set up systems of the history of organic beings which later discoveries have shown to be false. It was formerly thought that land animals and land plants had appeared first in the Carboniferous period; since then they have been found in the earlier Devonian system. Before the year 1824, many persons thought there could be no doubt that reptiles first existed in the Permian period; in the course of ten years it had been proved that they existed in the Carboniferous age, and even before that time. Before the year 1818 every one thought that the earliest remains of warm - blooded animals occurred in strata of the Cainozoic period; since then they have been found in the Oolitic, and

1 See Cotta, Geol. Bilder, p. 284. Cf. Pfaff, Grundriss, p. 380.

2 Vestiges of the Natural History of Creation. It is supposed that the Telerpeton Elginense-a lizard-like reptile-is a land animal; cf. Murchison, Siluria, p. 254. Lyell, Elements of Geology, p. 533. Latterly, however, it has been decided that the sandstone in which it was found belongs to the Triassic system (New Red Sandstone), and not to the Devonian system (Old Red Sandstone). Athenæum, 1863, Jan. 31, p. 153. Cf. Ausland, 1863, pp. 144, 192.

even in the Triassic systems, that is to say, in the Mesozoic period. It is because of this that Lyell asserts his conviction that in some ways science is only on the threshold of discovery as to the order in which the separate classes of organisms appear in the strata; and that in the second half of the century, as in the first, it will repeatedly find itself compelled to modify its previous theories.

We may also derive a good deal of information about the condition of the earth's surface in the earlier periods, about the distribution of land and water, climate, and so on, from the position and nature of the separate strata and from their organic contents; but this information must be very untrustworthy on account of the incompleteness and uncertainty of the materials on which the premises are founded. The maps which have been drawn of different parts of the earth's surface, as it appeared in the earlier periods, must in many cases be considered as merely hypothetical sketches, and fancy has more part than science

1 Lyell, Elements of Geology, ii. Fraas, Vor der Sündfluth, pp. 214, 255. The common theory, which rested principally on Murchison's investigations (Siluria, pp. 21, 469), that the Silurian system contains the remains of the first existing organisms, has also been attacked in recent years. It was supposed in 1864 that a gigantic extinct species of foraminifera, which was named Eozoon Canadense, had been discovered in Canada in the Laurentian strata, which are thought to be older than the oldest Silurian strata, and also in the gneiss of the Bohemian Forest, in Scotland, and in Sweden. See Lyell, Athenæum, 1864, Sept. 17. Murchison, Athenæum, 1865, Sept. 16. Dawson, The Dawn of Life; cf. his Nature and the Bible (Germ. tr. Gütersloh, 1877). But the organic nature of this fossil is denied on good grounds by others. Cf. Ferd. Römer, Ueber die ältesten Formen des organischen Lebens auf der Erde, Berlin 1869, p. 34. Pfaff, Grundriss, p. 224.

* See Huxley's remark above, p. 56. Cf. S. Zaddach, Die ältere Tertiärzeit ein Bild aus der Entwicklungsgeschichte der Erde, Berlin 1869. G. Berendt, Geognostische Blicke in alt-Preussen's Urzeit, Berlin 1872.

in the descriptions and pictures of primæval landscapes which are found in the popular expositions of geology written by Cotta, Fraas, and others. The opposition between the "Quietists" and "Convulsionists," which I mentioned above, is specially marked in the discussion as to the manner in which the fossiliferous strata were formed. Some assume that the strata were all formed just as strata of mud and sand are formed now; while others think that unusual catastrophes produced wide effects. Probably both methods of formation took place; it is not necessary to decide which had the greater result.

The animals which were attached to the groundbanks of shells, for instance-were destroyed by the gradual deposit of the surrounding strata. In other cases, sudden events, such as changes in the sea-level, escapes of gases, etc., seem to have killed masses of animals. A volcanic outburst in the middle of the sea, which was recently observed near Sicily, killed an immense number of sea-animals all round it. Similar events no doubt caused the destruction of the animals which we find petrified in masses, although they were easily able to move away, and thus escape. Buckland observes, speaking of a discovery of fossil fish: "The circumstances under which the fossil fishes are found at Monte Baldo seem to indicate that they perished suddenly on arriving at a part of the then existing seas, which was rendered noxious by the volcanic agency, of which the adjacent basaltic rocks afford abundant evidence. The skeletons of these fish lie parallel to the lamina of the strata of the calcareous slate; they

1 Cf. Cotta, Geol. Bilder, p. 257.

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are always entire, and so closely packed on one another, that many individuals are often contained in a single block. All these fishes must have died suddenly

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on this fatal spot, and have been speedily buried in the calcareous sediment then in the course of deposition. From the fact that certain individuals have even preserved traces of colour upon their skin, we are certain that they were entombed before decomposition of their soft parts had taken place. In the same manner also we may imagine deposits from muddy water, mixed perhaps with noxious gases, to have formed by their sediments a succession of thick beds of marl and clay, and at the same time to have destroyed, not only the testacea and lower orders of animals inhabiting the bottom, but also the higher orders of marine creatures within the regions thus invaded."1

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Most of the plants which are found petrified in the coal measures are land plants, especially tree-like ferns, and trees which existed between these and pines. These plants seem to have been partly torn from the then existing woods by floods, and to have been collected together at the bottom of lakes, rivers, and in seas or in narrow gulfs, and then transformed into coal. Probably most of the beds of coal were formed on the spot where the trees and plants had grown; the vegetation first passed into the condition of a peat moss, was then flooded by the sea in consequence of the sinking of the land, and was covered with a layer of mud and sand; by a subsequent elevation the mud was converted into dry land, and was fitted to produce a new forest, which then 'Geology and Mineralogy, etc., pp. 123, 124.

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process of time again became a peat moss. By the recurrence of this process the alternate layers of coal, sandstone, and slate were formed, which constitute the strata of the Carboniferous

period.1

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Our ideas as to the method in which the formation of the strata took place will influence our estimate of the period of time which elapsed during that formation. Geologists are unanimous with a few unimportant exceptions-in saying that a very long time must have elapsed before all the strata, many of which are in places several thousand feet thick, attained their present form. I quote a few of the calculations which have been made on points of detail, partly in order to show you in some measure what a very long time" means, and partly to prove how uncertain any attempt to express this very long time in figures must be. The time which elapsed between the beginning of the carboniferous age, which only constitutes one of the divisions of the Paleozoic period, and the recent period, is supposed by Arago to have been 313,600 years; G. Bischof estimates it at 1,300,000 years in one passage, and in another says it may have been 9 millions of years. Quenstedt makes the following calculation: "In order to form the Saarbrück coalbeds, which are 400 feet thick, a mountain of wood 2400 feet high would have been needed, supposing them to have been formed of vegetable matter. Now we know that our forests hardly produce a layer of

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2

Pfaff, Grundriss, p. 270 seq. Cotta, Geol. Bilder, p. 240 seq. Bischof, Lehrb. etc. (1st ed.), ii. 1814 (2nd ed. i. 745).

2 Burmeister, Geschichte der Schöpfung. p. 135.

3 Sonst und Jetzt, p. 170.

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