special and characteristic assemblages of animals and plants; but, in a general way, each subdivision of each formation has its own peculiar fossils, by which it may be recognised by a skilled worker in paleontology. Whenever, for instance, we meet in Britain with the fossils known as Graptolites, we may be sure that we are dealing with Silurian Rocks. We may, however, go much further than this. If the Graptolites belong to certain genera, we may be sure that we are dealing with Lower Silurian Rocks. Furthermore, if certain special forms are present, we may be even able to say to what exact part or subdivision of the Lower Silurian series they belong.

All these conclusions, however, would have to be accompanied by a tacit but well-understood reservation. No Graptolites have ever been found in Britain out of rocks known upon other grounds to be Silurian ; but there is no reason why they might not at any time be found in younger deposits. In the same way, the species and genera which we now regard as characteristic of the Lower Silurians, might at any time be found to have survived into the Upper Silurian period. We should never forget, therefore, in determining the age of a rock by paleontological evidence alone, that we are always reasoning upon generalisations which are the result of experience alone, and which may at any time be overthrown by fresh discoveries.

CHRONOLOGICAL SUCCESSION OF THE AQUEOUS Rocks.

As the result of observations made upon the superposition of rocks in different localities, from their mineral characters, and from their included fossils, geologists have been able to divide the entire stratified series into a number of different divisions or formations, each characterised by a general uniformity of mineral composition, and by a special and peculiar assemblage of organic forms. Each of these primary groups is in turn divided into a series of smaller divisions, characterised and distinguished in the same way. It is not pretended for a moment that all these primary rock-groups can anywhere be seen surmounting one another regularly. There is no region upon the earth where all the stratified formations can be seen together; and, even when most of them occur in the same country, they can nowhere be seen all succeeding each other in their regular and uninterrupted succession. The reason of this is obvious. There are many places-to take a single example-where one may see the Silurian Rocks, the Old Red Sandstone, and the Carboniferous Rocks succeeding one an

other regularly, and in their proper order. This is because the particular region where this occurs was always submerged beneath the sea while these formations were being deposited. There are, however, many more localities in which one would find the Carboniferous Rocks resting unconformably upon the Silurians without the intervention of any strata which could be referred to the Old Red Sandstone. This might arise from one of two causes: 1. The Silurians might have been elevated above the sea immediately after their deposition, so as to form dry land during the whole of the Old Red period, in which case, of course, no strata of the age of the Old Red Sandstone could possibly be deposited. 2. The Old Red Sandstone might have been deposited upon the Silurian, and then the whole might have been elevated above the sea, and subjected to an amount of denudation sufficient to remove the Old Red Sandstone entirely. In this case, when the land was again submerged, the Carboniferous Rocks, or any younger formation, might be deposited directly upon Silurian strata. From one

or other of these causes, then, or from subsequent disturbances and denudations, it happens that we can rarely find many of the primary formations following one another consecutively and in their regular order.

In no case, however, do we ever find the Old Red Sandstone resting upon the Carboniferous, or the Silurian Rocks reposing on the Old Red. We have therefore, by a comparison of many different areas, an established order of succession of the stratified formations, as shown in the subjoined ideal section of the crust of the earth (fig. 2).

The main subdivisions of the Stratified Rocks are known by the following names :—

Of these primary groups, the Laurentian, Cambrian, Silurian, Devonian, Carboniferous, and Permian are collectively grouped together under the name of Primary or Palæozoic Rocks (Gr. palaios, ancient; zoe, life), because of the entire divergence of their animals and plants from any now existing upon the globe. The Triassic, Jurassic, and Cretaceous systems are grouped together as the Secondary or Mesozoic formations (Gr. mesos, intermediate; zoe, life), because their organic remains are intermediate between those of the Palæozoic period, and those of more modern strata. The Eocene, Miocene, Pliocene, and Post-tertiary Rocks are grouped together under the head of Tertiary or Kainozoic Rocks (Gr. kainos, new; zoe, life), because their organic remains approximate in character to those now existing upon the globe.

CHAPTER III.

CONTEMPORANEITY OF STRATA AND
GEOLOGICAL CONTINUITY.

WHEN groups of beds in different parts of the earth's surface, however widely separated from one another, contain the same fossils, or rather an assemblage of fossils in which many identical forms occur, they are ordinarily said to be "contemporaneous;" that is to say, they are ordinarily supposed to belong to the same geological period, and to have been formed at the same time in the history of the earth. They would therefore be unhesitatingly regarded as "geological equivalents," and would be classed as Silurian, Devonian, Carboniferous, and so on. It is to be remembered, however, that it is not necessary, to establish such a degree of equivalency between widely separated groups of strata, that the fossils of each should be to any great extent specifically identical. It is sufficient that, whilst some few species are identical in both, the majority of the fossils should be "representative forms," or, in other words, nearly allied species. It will be shown, however, that groups. of strata widely removed from one another in point of distance can only exceptionally be "contemporaneous," in the strict sense of this term. On the contrary, in so far as we can judge from the known facts of the present distribution of living beings, the occurrence of exactly the same fossils in beds far removed from one another is primâ facie evidence, that the strata are

not exactly contemporaneous, but that they succeeded one another in point of time, though by no long interval geologically speaking.

Most of the facts bearing upon this question may be elicited by a consideration of such a widely extended and well-known formation as the Mountain Limestone or Sub-Carboniferous Limestone. This formation occurs in localities as remote from one another as Europe, Central Asia, North America, South America, and Australia; and it is characterised by an assemblage of well-marked fossils, amongst which Brachiopods belonging to the genus Producta may be specially singled out. Now, if we believe that the Carboniferous Limestone in all these widely distant localities was strictly contemporaneous, we should be compelled to admit the existence of an ocean embracing all these points, and, in spite of its enormous extent, so uniform in temperature, depth, and the other conditions of marine life, that beings either the same or very nearly the same inhabited it from end to end. We can, however, point to no such uniformity of conditions and consequent uniformity of life over any such vast area at the present day; and we have therefore no right to assume that this is the true explanation of the facts. Indeed, this explanation would almost necessarily lead us to the now abandoned theory that each period in geological history was characterised by a special group of organisms spreading over the whole globe, and that there took place at the close of each period a general destruction of all existing forms of life, and a fresh creation of the new forms characteristic of the next period.

In our inability, then, to accept this view, we must seek for some other explanation of the observed facts. The most probable view, and the one which is supported most strongly both by what we see at the present day and by what we learn from numerous examples in past time, is this:-The Carboniferous Limestone was not deposited all over the world in one given period, by one sea, or at exactly the same time; so that it cannot be said to be strictly "contemporaneous" wherever it is found. This would imply a uniformity of conditions over vast distances, such as exists nowhere at the present day, and such as we have no right to assume ever existed. On the contrary, the deposition of the Carboniferous Limestone must have first taken place in one comparatively limited area-say in Europe-where fitting conditions were present both for the animals which characterise it, and for the formation of beds of its peculiar mineral and physical characters. How wide this area may have been, signifies very little. It may have been as