The evidence is similar to that obtained by Schliemann on the site of Troy, where, in digging through successive layers of _débris_, he found the objects deposited by successive occupants of the site, from the time of the Roman Empire back to the earliest tribes, whose flint weapons and the ashes of their fires rest on the original surface of the ground.
Let us now tabulate the whole geological succession with the history of animals and plants associated with it:
Kainozoic. { Modern, { Pleistocene, Angiosperms and { Pliocene, Palms dominant. { Miocene, { Eocene.
Mesozoic. { Cretaceous, Cycads and Pines { Jurassic, dominant. { Triassic.
Age of Amphibians and Fishes. Age of Invertebrates.
Palæozoic. { Permian, Acrogens and { Carboniferous, Gymnosperms { Erian, dominant. { Silurian, { Ordovician, { Cambrian, { Huronian (Upper).
Eozoic. { Huronian (Lower), Protogens and Algæ. { Upper Laurentian, { Middle Laurentian, { Lower Laurentian.
It will be observed, since only the latest of the systems of formations in this table belongs to the period of human history, that the whole lapse of time embraced in the table must be enormous. If we suppose the modern period to have continued for say ten thousand years, and each of the others to have been equal to it, we shall require two hundred thousand years for the whole. There is, however, reason to believe, from the great thickness of the formations and the slowness of the deposition of many of them in the older systems, that they must have required vastly greater time. Taking these criteria into account, it has been estimated that the time-ratios for the first three great ages may be as one for the Kainozoic to three for the Mesozoic and twelve for the Palæozoic, with as much for the Eozoic as for the Palæozoic. This is Dana's estimate. Another, by Hull and Houghton, gives the following ratios: Azoic, 34·3 per cent.; Palæozoic, 42·5 per cent.; Mesozoic and Kainozoic, 23·2 per cent. It is further held that the modern period is much shorter than the other periods of the Kainozoic, so that our geological table may have to be measured by millions of years instead of thousands.
We cannot, however, attach any certain and definite value in years to geological time, but must content ourselves with the general statement that it has been vastly long in comparison to that covered by human history.