362 MERRILL — DISINTEGRATION AND DECOMPOSITION OF DIABASE. 



Columbia, or a residual cla}'', as in the more superficial portions of the 

 saprolitic deposits to the southward. In certain cases, or among certain 

 classes of rocks, the decom})osition proceeds at so rapid a rate that there 

 is scarcely an}" apparent prelimi iiar}" disintegration. Local circumstances 

 and character of rock masses being the same, Ave are, however, apparently 

 safe in assuming that in warm and moist climates decomposition follows 

 so closely upon disintegration as to form the more conspicuous feature 

 of the phenomenon, Avhile in dry regions, or those subject to energetic 

 frost action, mechanical processes prevail and disintegration exceeds de- 

 composition. I can but regard this point as one of considerable geolog- 

 ical significance. The source of certain materials forming secondary 

 rocks being known, it is possible that fronj their condition, as regards 

 decomposition, Ave may be able to draw some conclusions as to existing 

 climatic conditions at the time of their formation ; or if not of climatic 

 conditions, at least some idea as to relative ra[)idity of formation, an 

 arkose indicating either that physical agencies due to climatic conditions 

 prevailed, or else that the rapidity of disintegration, transportation and 

 deposition was such that chemical agencies had little cliance to get in 

 their work. 



These remarks are intended to be applicable mainly to silicious rocks. 



