STRATIFIED ROCKS. 207 



but is now known to be forming at present in deep seas. 

 But since, both now and in former times, the same kind 

 of deposits formed over wide areas, rocks of similar kind 

 (for example, sandstones of similar grain and color), and 

 especially a group of similar rocks, in' contiguous locali- 

 ties, are probably of the same age. But in widely sepa- 

 rated localities, as, for example, in different continents, 

 we can not use this method. To conclude that rocks are 

 of the same age, because they are of similar grain, color, 

 or composition, would almost certainly lead us astray. 



Comparison of Fossils. This is the most universal 

 and valuable means of comparison of rocks in all parts of 

 the world. If we find a general similarity of species, we 

 conclude that the rocks belong to the same age. But we 

 must make due allowance 1. For difference of conditions 

 of deposit, whether shore-deposit or deep-sea deposit, 

 whether fresh-water or marine. 2. We must also make 

 due allowance for geographical diversity. We must ex- 

 pect, in fossils of rocks in different continents, not abso- 

 lute identity, but only general similarity. We shall find 

 little difficulty in applying this, until we come to the 

 Tertiary. But here we have another principle to help us, 

 viz., the percentage of living invertebrates found in the 

 rock. Vertebrate, and especially mammalian species, may 

 be used in the Tertiary in much the same way as all species 

 in tho lower rocks. 



Construction of Chronology. By application of 

 these methods, geologists in all countries, working to- 

 gether, have gradually made a nearly complete chronol- 

 ogy. Breaks in one country are filled by strata in an- 

 other. But a really complete chronology can not be 

 expected until the whole surface of the earth has been 

 studied, and perhaps not even then, for some missing 

 links are probably concealed beneath the sea. 



Divisions and Subdivisions. The next task is to 

 divide and subdivide the whole into primary and second- 



