Introduction to Geology, 



69 



" Deposits originating in the way we have described must 

 necessarily be of variable thickness, and liable to every possible 

 modification from the action of mere local causes. Any useful 

 classification of their component beds would, perhaps, never 

 have been effected, had not the organic remains, preserved in 

 them, exhibited an extraordinary uniformity of character and 

 arrangement. An accurate examination of these spoils has, 

 therefore, supplied us with the means of establishing analogies 

 between phenomena which otherwise must have appeared en- 

 tirely unconnected." 



These deposits seldom appear consolidated in the form of 

 rocks, but generally consist of varieties of clay, marl, and 

 sand, with occasional concretionary masses. In this section 

 also occur two or three alternations of fresh-water or lacustrine 

 beds; that is, of deposits, which, from the numerous shells 

 they contain, resembling the Testacea of lakes and rivers, are 

 judged to have originated in fresh water. In Bavaria they are 

 stated to contain perfect beds of coal and iron-stone. In France 

 this class seems to have been carried somewhat higher than 

 in the English series. The latter comprises the plastic clay 

 and its accompanying sands, the London clay, the upper 

 marine or mixed formation of the Isle of Wight interposed 

 between the fresh-water strata, the shelly crag of Suffolk and 

 Norfolk ; and, above all these, particularly in the south-east and 

 eastern counties, appears a vast irregular accumulation of 

 debris, or water-worn and transported fragments of all the 

 preceding formations, known by the name of diluvium. 



Dr. Buckland noticed the remarkable occurrence of insu- 

 lated portions of tertiary strata on the summits of the Savoy 

 Alps, at elevations of more than 10,000 ft. above the level of 

 the sea ; and the observations of geologists have now fully de- 

 termined the fact, that tertiary formations exist in every quarter 

 of the globe, and differ in few essential respects from those 

 in this country. 



17 



HIgli Down. 



Horizontal Strata. 



fertfcaJ Strata of Alum Bay, 

 Isle of fiiglit. 



We have selected an illustrative section of the western ex- 

 tremity of the Isle of Wight (Ji^, 17.)j where the entire series 



F 3 



