§ 10. GEOLOGICAL MUTATIONS. 39 



4th. The Tertiary Strata. — These strata lie upon, and 

 fill up, depressions or basins of the chalk, and other secondary 

 rocks; they consist of the detritus of more ancient beds, and 

 of the relics of shells, plants, zoophytes, Crustacea, fishes, 

 &c. : and in them, with but one exception, the bones of 

 mammalia first appear. 



oth. Alluvial Deposits* — Of a later formation than the 

 tertiary, are those irregular accumulations of alluvial or 

 water-worn and drifted materials, which are spread over 

 the surface of almost every country. In the newest of these 

 beds are found remains of existing races of animals and 

 plants, which, in the most ancient, are associated with those 

 of extinct genera and species. 



Even this slight examination of the strata affords con- 

 vincing proofs of a former condition of animated nature, 

 widely different from the present. We have evidence of a 

 succession of periods of unknown duration, in which both 

 the land and the sea teemed with forms of existence that 

 have successively disappeared and given place to others ; 

 and these again to new races, more nearly related to those 

 which now inhabit the earth, till at length traces of existing 

 species appear. 



10. Geological mutations. — From this view of the 

 mineral structure of our planet we learn, at least so far as 

 the limited powers of man can penetrate into the history of 

 the past, that the distribution of land and water on the 

 earth's surface has been undergoing perpetual mutation ; 

 yet, that through a vast period of time, its physical condition 

 has not materially differed from the present. We find that 

 the dry land has been clothed with vegetation, and tenanted 

 by appropriate inhabitants ; and that the seas and the 



* The term diluvial is commonly employed to denote the most 

 ancient of these deposits, and that of alluvial the more recent, in 

 which existing species of animals only occur. 



