THE GEOLOGICAL EPOCHS: AND THE TIME RANGE OF FOSSILS.

Superposition of Strata.—

Fossils are chiefly found in rocks which have been formed of sediments laid down in water, such as sandstone, shale and most limestones. These rocks, broadly speaking, have been deposited in a horizontal position, though really slightly inclined from shore to deep-water. One layer has been formed above another, so that the oldest layer is at the bottom, and the newest at the top, of the series (Fig. 11). Let us, for instance, examine a cliff showing three layers: the lower, a sandstone, we will Call A; the intermediate, a shale or clay bed, B; and the uppermost, a limestone or marl, C (Fig. 12). In forming a conclusion about the relative ages of the beds, we shall find that A is always older than B, and B than C, provided no disturbance of the strata has taken place. For instance, the beds once horizontally deposited may have been curved and folded over, or even broken and thrust out of place, within limited areas; but occurrences like these are extremely rare. Moreover, an examination of the surrounding country, or of deep cuttings in the neighbourhood, will tell us if there is any probability of this inversion of strata having taken place.

This law of superposition holds good throughout the mass of sedimentary rocks forming the crust of the earth.

(1). Thus, the position of the strata shows the relative ages of the beds.

Differences in Fossil Faunas.—

Turning once again to our ideal cliff section, if we examine the fossils obtained from bed A, we shall find them differing in the number of kinds or species common to the other beds above and below. Thus, there will be more species alike in beds A and B or in B and C. In other words the faunas of A and B are more nearly related than those of A and C. This is explained by the fact that there is a gradual change in specific forms as we pass through the time series of strata from below upwards; so that the nearer one collecting platform is to another, as a rule, the stronger is the community of species.

Guide Fossils.—

Certain kinds of fossils are typical of particular formations. They are known as guide fossils, and by their occurrence help us to gain some idea of the approximate age of rocks widely separated by ocean and continent. Thus we find fossils typical of the Middle Devonian rocks in Europe, which also occur in parts of Australia, and we therefore conclude that the Australian rocks containing those particular fossils belong to the same formation, and are nearly of the same age.

(2). The included fossils, therefore, give evidence of the age of the beds.

Value of Lithological Evidence.—

The test of age by rock-structure has a more restricted use, but is of value when taken in conjunction with the sequence of the strata and the character of their included fossils.

To explain both the valuable and the uncertain elements of this last method as a determinant of age, we may cite, for instance, the Upper Ordovician slates of Victoria and New South Wales as an example of uniform rock formation; whilst the yellow mudstones and the grey limestones of the Upper Silurian (Yeringian series) of the same states, are instances of diverse lithological structures in strata of similar age. A reference in the latter case to the assemblages of fossils found therein, speedily settles the question.

(3). Hence, the structure and composition of the rocks (lithology), gives only partial evidence in regard to age.

Strata Vertically Arranged.—

The Stratigraphical Series of fossiliferous sediments comprises bedded rocks from all parts of the world, which geologists arrange in a vertical column according to age.

A general computation of such a column for the fossiliferous rocks of Europe gives a thickness of about 14 miles. This is equivalent to a mass of strata lying edgewise from Melbourne to Ringwood. The Australian sediments form a much thicker pile of rocks, for they can hardly fall short of 37 miles, or nearly the distance from Melbourne to Healesville.

This vertical column of strata was formed during three great eras of time. The oldest is called the Primary or Palaeozoic (“ancient life”), in which the animals and plants are of primitive types. This is followed by the Secondary or Mesozoic (“middle life”), in which the animals and plants are intermediate in character between the Palaeozoic and the later, Cainozoic. The third era is the Tertiary or Cainozoic (“recent life”), in which the animals and plants are most nearly allied to living forms. These great periods are further subdivided into epochs, as the Silurian epoch; and these again into stages, as the Yeringian stage.

Vertical Column of Fossiliferous Strata, Australia.

1.—The classification of the Cainozoics as employed here is virtually the same as given by McCoy in connection with his work for the Victorian Geological Survey. The writer has obtained further evidence to support these conclusions from special studies in the groups of the cetacea, mollusca and the protozoa. The alternative classification of the cainozoics as given by one or two later authors, introducing the useful local terminology of Hall and Pritchard for the various stages or assises is as follows:—

2.—Or Permo-carboniferous. As the series is held by some authorities to partake of the faunas of both epochs, it is preferable to use the shorter word, which moreover gives the natural sequence. There is, however, strong evidence in favour of using the term Permian for this important series.3.—Mr. W. S. Dun regards the Lepidodendron beds of W. Australia, New South Wales and Queensland as of Upper Devonian age. There is no doubt, from a broad view of the whole question as to the respective age of these beds in Australia, that the one series is continuous, and probably represents the Upper Devonian and the Lower Carboniferous of the northern hemisphere.4.—These limestones contain a fauna of brachiopods and corals which, at present, seems to point to the series as intermediate between the older Silurian and the Upper Ordovician.

Vertical Column of Fossiliferous Strata, New Zealand.

Note 1.—Based for the most part, but with some slight modifications,
on Prof. J. Park’s classification in “Geology of New Zealand,” 1910.