Evolution of the Earth. 97 



IV. Organic. 



a. Vegetable deposits, forming peat-bogs and coal-beds. 



b. Shell deposits of extinct animals, forming limestone 



and marble. 



c. Foraminifera deposits, forming chalk-beds and marl. 



d. Action of coral polyps, forming coral reefs, islands, 



and peninsulas. 



The Order of Geological Succession. Subsequent 

 to the period of the primeval igneous rocks, geologists re- 

 cognize a succession of four great eras, three of them being 

 divided and sub-divided into subordinate periods of varia- 

 ble duration. Tabularly represented, in out-line merely, 

 these geological time-divisions appear as follows : 



I. Azoic or Archosan. 



II. Palaeozoic, or Primary. 



a. Cambrian. 



b. Silurian. 



c. Devonian. 



d. Carboniferous. 



e. Permian. 



III. Mesozoic, or Secondary. 



a. Triassic. 



b. Jurassic. 



c. Cretaceous. 



IV. Cenozoic, or Tertiary. 



a. Eocene. 



b. Miocene. 



c. Pliocene. 



d. Post-Pliocene. 



e. Recent, (Psychozoic.) 



Tracing the order of Geological succession, in the lower 

 non-sedimentary rocks, we, of course, find no remains of 

 organic life. Neither have any been discovered in the sed- 

 imentary rocks of the earliest or Azoic period, though it is 

 not improbable that early fragile forms may have existed, 

 that left no perceptible trace. The subsequent order of geo- 

 logical succession is determined mainly by the order of su- 

 perposition of the strata, — on the logical supposition that 

 the undermost stratified rocks are the oldest. As we go 

 back toward the older rocks, we find that the fossils be- 

 come more and more dissimilar to those forms of an- 



