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that therefore formations containing land and fresh water fossils must be compared with one another, and the same with formations containing sea fossils.'

The employment of fossils as a means of geological calculation is, as has been observed,' quite as justifiable as the mode of proceeding which obtains in other provinces of human knowledge, e.g. in the history of architecture. Monuments which are historically well authenticated are the foundation for researches into this branch of knowledge. From these the art historian collects the special characteristics of a particular style or period, round arches, pointed arches, for instance, and so on, and from the presence of these characteristics he draws inferences concerning the date of those monuments, which are not historically authenticated. Surely we may say that a rule drawn from the works of man is less certain than one drawn from the works of nature.

No doubt in many cases great difficulties attend the application of this rule in geology. It is often difficult, sometimes impossible, to define the boundary between two superimposed strata; we find strata which in some localities appear to belong to the lower, in others to the upper formation, and which are therefore sometimes ascribed to one and sometimes to the other. It is often still more difficult to decide which strata in different countries are to be considered as parallel or equivalent to each other, and ought therefore to be ascribed to the same period. It is true that in every

1 Cotta, Geol. Bilder, p. 185. Pfaff, Schöpfungsgesch. p. 5.
2 J. Probst in the Tübingen Quartalschr. 1866, p. 140.

Vogt, Lehrb. der Geologie, ii. p. 390.

case in which the older formations have been examined, the fossils have been found to be practically similar; but in the more recent formations they afford very few points of comparison in the different localities; sometimes the separate strata are even found in quite a different order in different regions.' But, on the whole, the following arrangement of the strata may, according to the unanimous opinion of all modern geologists, be considered as correct.

The first class comprises the lowest rocks, those resting immediately on the primitive rocks, gneiss, mica schist, and chlorite slate, in which no fossils are found, and which, it is therefore supposed, were deposited at a time when there were no organic beings on the earth; these formations are therefore called those of the Azoic period. The fossiliferous strata are denominated those of the Palaeozoic, Mesozoic, and Cainozoic Ages-that is, formations belonging to the older, middle, and modern periods of organic life on the earth. (These four periods are also called the Archæolithic, Paleolithic, Mesolithic, and Cainolithic.) The formations made from deposits in the historical period may be reckoned as a fifth class, those of the Recent Age; they include coral islands, river deltas, sand hills, deposits of calc-sinter, turf beds, etc.2

1 Vogt, Op. cit. i. p. 561.

2 The following list of the stratified formations may be useful in showing the different names :

I. AZOIC PERIOD Mica Schist. 1. Gneiss

(1, 2).

(Laurentian System).

2. Chlorite Schist (Huronian System), (Cambrian System).

A. Transition Rocks

(1-4).

As the animals and plants whose remains are found in the strata of the separate systems must have lived on the earth before these strata were formed, we can obtain an approximate idea of the fauna and flora which belonged to the separate periods of the earth's history; it can only be approximate, not accurate, first because we do not know all the fossils which exist, and then because it does not follow that traces remain of all the organisms which existed. This much is certain, that the organic life on the earth has not always been the same; no trace is found in the later formations of

[blocks in formation]

Lyell, by whom the name "Eocene," etc., was introduced, has modified this division in the following way: he includes the eocene, miocene, and pliocene series in the Tertiary or Cainozoic period; and he calls everything more recent than the pliocene, post-tertiary. He then divides these post-tertiary formations into post-pliocene and recent. He calls those strata recent whose fossils, shells as well as mammals, belong to the still existing species; on the other hand, he calls those deposits, in which the shells belong to existing, but the mammals for the most part to extinct species, post-pliocene. Others call the lowest strata of the miocene, oligocene;" others, everything which lies above the eocene, “neocene," or "neogene."

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several of the organisms belonging to the older formations; they must therefore have died out in the earlier period. Again, there is no trace in the earlier formations of several of the organisms belonging to the later formations, therefore they cannot have existed in the earlier period. Geological research will not justify us in assuming anything beyond this general rule. Many of the details are uncertain and disputed. Some geologists say, for instance, that every formation has its separate fauna and flora, and that the same kind of animal or plant is never, or very rarely, found in two succeeding formations; and that it must therefore be assumed that in the course of ages organic life repeatedly became extinct on the earth, and was again restored.1 Others maintain that since the first appearance of organic beings, single species have now and then become extinct, and others have come into existence, but that such gradual transformation of organic life was caused by the existence and filling up of gaps, and that the thread was never entirely broken. More recently a theory, which we shall discuss at length later on, has been much in favour, namely, that all the later plants and animals, including all at present existing, are descended from those of the earliest period, so that it is not possible to classify the fauna and flora as belonging specially to separate periods.3

2

In comparing with one another the forms of plants and animals belonging to different periods, as we get to

1 Cuvier, A. Brongniart, A. d'Orbigny, Agassiz (cf. Les animaux et les plantes aux époques géologiques, in the Revue des Cours Scientifiques, Paris 1868, No. 49 seq.), Murchison (Siluria, p. 461), and others.

2 Prevost, de Blainville, Schlotheim, Bronn, and others. 3 Cf. Zittel in the Historisch Taschenb. p. 167.

know them by means of the fossils, we find that the earliest differ most, and the later ones least, from the present fauna and flora. Speaking generally, therefore, we may assume that a development of plants and animals from more imperfect to more perfect forms has taken place, however that development may be explained. The earliest formations contain scarcely any but the remains of creatures of a low organization; flowerless plants, corals, molluscs, and articulata. There are very few signs of fish and reptiles, and as far as is known at present, no signs of birds or mammals. In the succeeding strata more highly organized plants and animals are found; in the Carboniferous period there are some conifers, many fish, a few reptiles; in the Triassic period, many reptiles, a few birds, and mammals; in the Oolitic period, a few dicotyledon plants and more mammals; and in the Tertiary period, many dicotyledon plants and mammals. In all cases the lower

organisms of the great separate divisions of the animal and vegetable worlds appear first, and the higher organisms later. Thus, of the radiata, the crinoidea appeared first; of the fish, first the tailed ganoid and the placoid; of the reptiles, the saurians; of the birds, first the marsh birds and tufted birds; of the mammals,

the opossums and cetacea. The organic forms differ

most from those now existing in the earliest strata, and the difference diminishes steadily all through the more recent deposits. Some of the animals and plants in the older formations belong to classes which are quite extinct; later they differ from those now existing in genus only; later still only in species. Some still existing kinds of larger animals appear first as fossils

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