164 MEANING OE THE SYSTEM. 



the deeper a stratum comes in the series, that is, the earlier it 

 appears in the history of the earth, so much the more its fauna and 

 flora differ from those of the present time. The more nearly one 

 stratum follows another in the series, the closer the relationship 

 between their respective fossils. Every sedimentary formation 

 possesses characteristic fossils which appear very frequently; and 

 from these, taking into account the succession of strata and the 

 petrographic characters of the rocks, the place occupied by the 

 stratum in the geological system can be denned with tolerable 

 accuracy. 



Without doubt the characters of the fossils and the relative posi- 

 tions of the strata are the most important aids to the determination 

 of the geological age of the deposit ; at any rate they furnish a more 

 reliable criterion than does the structure of the rocks. The idea 

 entertained in earlier times that rocks of the same period always 

 possessed a similar, and rocks of a different period a dissimilar 

 structure, has lately been given up as erroneous. Stratified or 

 sedimentary deposits have arisen in every period under similar condi- 

 tions. In past times, as at the present time, they were caused by 

 the deposition of clay, of fine or coarse sand, of fine and coarse debris, 

 by chemical precipitation of carbonates and sulphates of lime and 

 magnesia, of silica and oxide of iron, and by accumulation of solid 

 animal and vegetable remains. These have become transformed only 

 in course of time into such hard rocks as argillaceous and calcareous 

 schists, limestone, sandstone, dolomite, and conglomerates of many 

 kinds ; as the result of many causes, such as mechanical pressure of 

 superincumbent masses, increase of temperature, internal chemical 

 processes, and so forth. 



Even though the peculiar structure of rocks may in many cases 

 afford good ground for conjecture as to the relative age, yet 

 it is certain that deposits of similar age may show an entirely 

 different petrographical character ; and, on the other hand, that 

 deposits of very different ages may have given rise to rock forma- 

 tions that can be scarcely or not at all distinguished from one 

 another. 



The old idea that deposits of the same age must everywhere contain 

 the same fossils, could only be maintained as long as geological inves- 

 tigations were confined to small districts. Similarly the idea, closely 

 connected with the former, that the various geological formations, 

 characterised by a series of definite strata, are entirely independent 

 of one another, no longer obtains credit. The various forma- 



