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 dejDosits, 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 Archivan. 



III. Mesozoic, or Secondary. 



a. Triassic. 



b. Jurassic. 



c. Cretaceous. 



IV. Cenozolc, 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 V)een 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. Tli(> 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- 



