STRATIFIED ROCKS. 05 



approach to living forms, first in families, then in genera, 

 and then in species. Species do not begin to be identical 

 with the living species until the Tertiary period, and 

 thence onward we have an increasing percentage, identical 

 with the living. 



Now, we determine that rocks belong to the same time, 

 all over the earth, by the general similarity of the fossil 

 species. We find difficulty in applying this rule only in 

 the Tertiary, because then the geographical diversity is 

 beginning to be so great as seriously to interfere with the 

 general similarity. But just here we begin to use another 

 principle, viz., the percentage of the fossil species still 

 living in the immediate vicinity. Similar percentage in- 

 dicates the same age greater percentage less age, and 

 less percentage greater age.* It is on these principles 

 that is based the classification of stratified rocks. 



SECTION II. CLASSIFICATION OF STRATIFIED ROCKS. 



Geology is a history. Stratified rocks are the leaves 

 of an historical book. Evidently, then, the true basis of 

 classification must be relative age. In classification, the 

 geologist has two objects in view : 1. To arrange all the 

 strata, from lowest to highest, in the order in which they 

 were formed. 2. Then to separate them into groups and 

 sub-groups for convenient treatment i. e., 1. To arrange 

 the leaves in the order in which they were written, so that 

 the story they contain may be read intelligently. 2. To 

 divide and subdivide into chapters and sections, deter- 

 mined by great events in the history. In a word, he must 

 make first a chronology, and then divide into eras, ages, 

 periods, etc. 



Chronology ; Order of Superposition. It is evi- 

 dent, from the manner in which sediments are formed, 



* The teacher should consult the larger work, for a complete state- 

 ment. 



