200 MINERALS AND GEOLOGY 



remains of which are entombed in so many of the sedimentary rocks. 

 The sediments now under process of deposition in our lakes, river- 

 estuaries, and seas, frequently enclose, it will be remembered, the more 

 durable parts, if not the entire forms, of various plants and animals, 

 amongst which aquatic types necessarily preponderate. The sedimen- 

 tary deposits of former geological periods have enclosed in like manner 

 various organic forms peculiar to those periods. . In the very lowest 

 or earliest formed deposits, it is true, no traces of organic types have 

 yet been met with, but above these beds, each group of strata holds 

 its own characteristic fossils. Regarded broadly, the higher groups 

 contain the higher organisms ; and many structural conditions which 

 are now embryonic or transitory, were manifested as adult or perma- 

 nent forms of development in the periods represented by lower groups. 

 Type after type lived through its allotted time, and then died out to 

 be replaced by other and in general by higher forms of life. These 

 facts are discussed more fully in Part IY. of this Essay, in which the 

 leading questions connected with the subject of Organic Remains come 

 imder review. For present purposes, it will be sufficient to observe 

 that by the careful study and comparison of these remains, geologists 

 have sub-divided the series of rock-masses of which the Earth's crust 

 or outer portion is composed, into a certain number of so-called For- 

 mations, each Formation representing an interval or period in the 

 ancient history of the Earth. These periods are thus made known 

 to us by the various rocks produced by aqueous and other agencies 

 during their continuance ; and by the organic remains, deriv d from 

 the living forms of the periods in question, which are enclosed in 

 these rocks. Each Formation, as already stated, holds its own 

 organic types ; although, when viewed apart from local distinctions, 

 consecutive Formations appear to merge into each other, as an 

 ordinary historic period blends insensibly with that which precedes 

 and that which follows it. This is the case in natural groupings or 

 classifications of all kinds : hard or sharply-defined lines being strictly 

 unrecognized by Nature. The divisions however adopted by geolo- 

 gists, although overlapping as it were at their common boundaries, 

 are distinct enough in the main ; and as some of these divisions are- 

 linked together more or less closely by the presence of certain related 

 types of life, as well as by the general absence of other types, a, 

 grouping of Formations into larger divisions, representing longer 



