70 Introduction to Geology. 



of tertiary strata are developed under tlie most interesting 

 circumstances. The present is not the time or place for ad- 

 verting more particularly to the extraordinary local disturbance 

 by which so many of the superior formations have acquired 

 a vertical position. 



General Observations on the Strata "which contain Organic 

 Remains, 



The word Formation is understood to apply to certain groups 

 of strata which have been associated together for the sake of 

 ready and simple classification, and of avoiding the perplexity 

 otherwise arising from their numerous subordinate beds. But 

 for this comprehensive arrangement, it would be difficult to 

 distinguish, with sufficient precision, the separate members of 

 the secondary strata. As each formation is, in general, made 

 up of several members, so also the latter are composed of an 

 infinite number of beds ; and those beds, again, of innumerable 

 finer lamina : but the whole series being found to possess some 

 common character, some obvious relations, or some general 

 similarity in organic accompaniments or circumstances of po- 

 sition, they are appropriately classed under one common deno- 

 mination. 



In this country these names are chiefly derived from some 

 predominant quality or structure; as the green or chloritic 

 sand, ferruginous sands, red sandstone, magnesian or dolomitic 

 limestone, gault, clunch, and oolites. To some of these de- 

 scriptive titles others have been superadded, which are derived 

 from certain localities where these formations are best de- 

 veloped; such as the Folkstone marl. Weald clay, Oxford 

 clay, Hastings sands, Woburn sands, and Shanklin sands. 

 Others are wholly derived from localities ; as the London clay, 

 Kimmeridge clay, Portland limestone, Petworth marble. Forest 

 marble, &c. 



As the formations comprehend many inferior beds, it must 

 often occur that they contain some whose mineralogical cha- 

 racters differ materially from those imported by their names. 

 For this reason a nomenclature derived from characters not 

 universal must be considered defective. Of this class is what 

 is termed the plastic clay formation, which chiefly consists of 

 sand. The Weald clay contains thick beds of sand, with lime- 

 stone and fresh-water marbles. We may add also the clays 

 and limestones of the ferruginous sands, the Kelloways rock, 

 the Oxford clay, &c. 



The practical geologist is often led to perceive frequent 

 transitions and modifications in the structure of the same beds ; 

 the clays often changing to indurated argillaceous rocks, shales, 



