Not only do they constitute the oldest stratified accumulations, but from the extreme divergence between their animals and plants and those now in existence, they may appropriately be considered as belonging to an "Old-Life" period of the world's history.

The Triassic, Jurassic, and Cretaceous systems are grouped together as the _Secondary_ or _Mesozoic_ formations (Gr.

_mesos_, intermediate; _zoe_, life); the organic remains of this "Middle-Life" period being, on the whole, intermediate in their characters between those of the palaeozoic epoch and those of more modern strata.

Lastly, the Eocene, Miocene, and Pliocene formations are grouped together as the _Tertiary_ or _Kainozoic_ rocks (Gr.

_kainos_, new; _zoe_, life); because they constitute a "New-Life" period, in which the organic remains approximate in character to those now existing upon the globe.