--------------------+---------------------+-------------------------- ANIMALS |SYSTEMS OF FORMATIONS| PLANTS --------------------+---------------------+-------------------------- | Kainozoic | | Modern | Age of Man and | Pleistocene | Angiosperms and Mammalia | Pliocene | Palms dominant | Miocene | | Eocene | --------------------+---------------------+-------------------------- | Mesozoic | | Cretaceous | Cycads and Pines Age of Reptiles | Jurassic | dominant | Triassic | --------------------+---------------------+-------------------------- | Palæozoic | | Permian | | Carboniferous | Age of Amphibians | Erian | Acrogens and Gymnosperms and Fishes | Silurian | dominant Age of Invertebrates| Ordovician | | Cambrian | | Huronian (Upper) | --------------------+---------------------+-------------------------- | Eozoic | | Huronian (Lower) | Age of Protozoa | Upper Laurentian | Protogens and Algæ | 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, 23.3 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.