Handbook of Paleontology 183 



THE FORMATIONS 



Sediments in the geologic past were deposited under 

 much the same conditions as those today and have be- 

 come the conglomerates, sandstones, shales, limestones 

 etc. constituting our geological formations. Animals and 

 plants in the past also were influenced by their environ- 

 ment much as today, forming plant and animal associa- 

 tions then as now and giving us the characteristic faunas 

 and floras of our formations. Certain types of fossils are 

 characteristic of sandstones ; others of limestones, shales 

 etc., varying as the conditions under which they lived 

 varied at the time of deposition of the sediments making 

 the rock. 



The conditions for the preservation of fossils in the 

 rocks and their significance is discussed in part 1 ; the 

 time scale and rock scale and the stratigraphic sequence 

 of sedimentary rocks in the introduction to this volume 

 pages 60, 61). Briefly, the normal sequence of undis- 

 turbed rock from older to younger, that is, from earliest 

 deposited to latest deposited, is termed the stratigraphic 

 sequence. In the division of the time and rock scales, the 

 era of time corresponds to a group of rocks. Eras are 

 the longest divisions of time used in geology and they are 

 separated by great unconformities and marked near their 

 close by times of extensive mountain building, bringing 

 about great changes in the environment with a consequent 

 striking effect upon the life of the times. Eras are com- 

 posed of periods of times with their corresponding sys- 

 tems of rocks. Periods of time are of very long duration 

 and systems of rocks are usually of great thickness. They 



