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lowish colored sand took place, which are at present to be 

 seen in various situations, protruding as it were from the 

 roofs of these excavations, and forming a singularly dis- 

 tinct contrast to the dark stained material into which they 

 are enclosed. From the circumstance that these appa- 

 rently isolated masses of sand never exhibit those undu- 

 lating lines and angles so characteristic of the other and 

 associate deposits of the kind, I should form the conclu- 

 sion that they had derived their origin from the surface 

 waters, slowly and gradually percolated through the nu- 

 merous thin seams or shrinkage fissures in the overlying 

 and disruptured clays ; for I can conceive of no other or 

 more simple process to account for their present appear- 

 Sometimes the rounded pebbles which constitute the 

 gravel, and which are at all times to be found in connec- 

 tion with these imbedded brown sands, are cemented firm- 

 ly together by a copious deposit of carbonate of lime, to 

 such a degree as to form extensive beds of calcareous con- 

 glomerate, or pudding-stone, the lime being derived from 

 the percolating waters containing it in solution ; but this 

 only occurs when the broken up strata of drab-colored 

 clay are either in their immediate vicinity, or directly 

 above. In other instances, the cementing ingredient is 

 the sulphuret of iron, which then renders the united mass 

 so exceedingly hard as to destroy the tools of the work- 

 men employed in its excavation. 



The marly clays of this formation, with their imbedded 

 contents, are sometimes subject to frequent and extensive 

 slides. They are of common occurrence all over the slop- 

 ing sides of the numerous and deeply cleft ravines which 

 open from the surface of the plain, down to the shores of 

 the diflerent streams. To an individual looking from an 

 eminence across one of these ravines to the hill-side be- 

 yond, it will most generally appear to be composed almost 

 entirely of a succession of irregularly formed terraces, 



