60 New York State Museum 



has been severe deformation in any region the order 

 of the strata may be reversed as has been shown in the 

 section on folding. To the original and normal se- 

 quence of strata from older to younger the term 

 siratigraphic sequence is applied. 



The relation of a bed of rock to the one above or 

 the one below is known as its relative age. When the 

 normal relation is altered fossils, when present, and 

 comparison with the same series elsewhere will deter- 

 mine the relative age. The age of a rock bed may 

 also be considered from the point of view of the place 

 it holds in the geologic time scale or its geologic age, 

 or from the point of view of its actual age in years. 

 Geologists all over the world have come to more or less 

 agreement upon a time seale, which is a definite suc- 

 cession of rock formations for the whole world, and 

 to these divisions names have been given (page 190). 

 Just as human history can be divided, so geologic his- 

 tory may b«: divided into eras, periods etc., and the dif- 

 ferent kind^ of rocks and rock structures can be re- 

 ferred to their respective geologic periods. The stu- 

 dent will find some variation in the usage of different 

 writers, especially in the minor divisions. The various 

 divisions have been named with no particular system. 

 The name maybe derived from some place or locality 

 as Jurassic period from the Jura mountains, Devonian 

 period from Devonshire in England where the rocks 

 were first studied, and Onondaga limestone from On- 

 ondaga county. The carbon, that is coal, content of 

 another system of rocks has given to them the name 

 Carboniferous (carbon-bearing) : and the Cretaceous 

 was named from the chalk rocks (Latin creta) preva- 

 lent in that period. Besides the scale of time, there is 



