7l8 STUDIES FOR STUDENTS 



sands and sandstones is due to this action. Through a 

 similar process of deoxidation ferrous sulphates may be con- 

 verted into sulphides, a process which undoubtedly takes place 

 in marine muds protected by the water from atmospheric action. 

 Hydration — the assumption of water — more commonly accom- 

 panies oxidation, and indeed is an almost constant accompani- 

 ment of rock decomposition, as may be observed in comparing 

 the total percentages of water in fresh and decomposed min- 

 erals and rocks, as given in any series of analyses. The amount 

 of water thus taken up is in some cases surprisingly large. This 

 assumption, provided it be not accompanied with an equal loss 

 of other constituents, is attended with an increase in bulk such 

 as may be quite appreciable. In cases where rock disintegra- 

 tion progresses without serious decomposition or surface erosion 

 a corresponding expansion must also take place. The Comte 

 de la Hure, as quoted by Branner,^ has expressed the opinion 

 that some of the hills of Brazil have actually increased in height 

 through this means. The present writer has calculated that the 

 transition of the granitic rock of the District of Columbia into 

 arable soil must be attended by an increase in bulk amounting 

 to 88 per cent. 



Hydration as a factor in rock disintegration is, in the 

 writer's opinion, of more importance than is ordinarily sup- 

 posed. Granitic rocks in the District of Columbia have been 

 shown'' to have become disintegrated for a depth of many feet 

 with loss of but some 13.46 per cent, of their chemical con- 

 stituents and with apparently but little change in their form 

 of combination. Aside from its state of disintegration the 

 newly-formed soil differs from the massive rock mainly in that 

 its feldspathic and other silicate constituents have undergone a 

 certain amount of hydration. Natural joint blocks of the rock 

 brought up from shafts excavated during the extension of the 

 city waterworks were, on casual inspection, sound and fresh. 

 It was noted, however, that on exposure to the atmosphere such 



' Bull. Geol. Soc. of America, Vol. VII, 1896, p. 284. 



* Merrill, Bull. Geol. Soc. of America, Vol. VI, p. 341, and Vol. VII, p. 357. 



