4. By the aid of the superposition of deposits and their organic remains, geology can divide the history of the earth into distinct periods. These periods are not separated by merely arbitrary boundaries, but to some extent mark important eras in the progress of our earth; though they usually pass into each other at their confines, and the nature of the evidence prevents us from ascertaining the precise length of the periods themselves, or the intervals in time which may separate the several monuments by which they are distinguished. The following table will serve to give an idea of the arrangement at present generally received, with some of the more important facts in the succession of animal and vegetable life, as connected with our present subject. It commences with the oldest periods known to geology, and gives in the animal and vegetable kingdoms the _first appearance_ of each class, with a few notes of the subsequent history of the principal forms. It must, however, be borne in mind that farther discoveries may extend some classes farther back than we at present know them, and that a more detailed table, descending to orders and families, would give a more precise view of the succession of life. Farther, the several geological formations would admit of much subdivision, and are represented locally by various kinds and different thicknesses of sediment.[142] TABULAR VIEW OF THE SUCCESSION OF GEOLOGICAL FORMATIONS AND ORGANIC REMAINS. ==================================================================== PERIODS. | SYSTEMS OF | CLASSES OF ANIMALS. | PLANTS. | FORMATIONS. | | -------------------------------------------------------------------- I. |Ancient Metamorphic |Eozoon and probably other|Graphite and EOZOIC |rocks of | Protozoa. |Iron Ores PERIOD. |Scandinavia, | |representing |Canada, etc. | |Vegetable | | |Matter. -------------------------------------------------------------------- II. |Cambrian. |_Radiata_--Hydrozoa, |Algae. PRIMARY | | Echinodermata | OR | | (Cystideans). | PALAEOZOIC| |_Mollusca_--Brachiopoda, | PERIOD. | | Lamellibranchiata, | | | Gasteropoda, Cephalopoda| | | (Bivalve and Univalve | | | Shell-fishes). | | |_Articulata_--Annelida, | | | Crustacea (Worms and | | | Soft Shell-fishes of the| | | lower grades). | | | | |Lower Silurian.