CHEMICAL CHANGES DURING DECOMPOSITION. 



507 



CHANGE IN CHEMICAL COMPOSITION OF THE ROCKS. 



So far as the change in the average chemical composition of rocks in 

 the belt of weathering is concerned, solution is largely the final determina- 

 tive factor: although, as has been seen, the amount and nature of the 

 material dissolved are dependent upon all the many factors' heretofore con- 

 sidered. By comparing the original chemical composition of rocks with the 

 chemical composition of the partly disintegrated and decomposed equivalent 

 rocks, one is able to obtain an approximate idea of the relative losses of the 

 elements. In order to compare the losses which the various elements have 

 undergone as a result of solution and transportation, it is necessary to 

 suppose that some one element is insoluble and is not abstracted at all. 

 Usually iron and aluminum have been chosen as the insoluble constituents. 

 With these as a standard in most cases, but in some cases other elements, 

 Merrill made the following calculations as to the losses of the elements for 

 various rocks:" 



Supposing the amount of iron to remain constant, for a disintegrated 

 and partly decomposed granite in the District of Columbia the original 

 composition and the losses of the constituents are as follows: 



Table I. — Analyse* of fresh and altered granite. 



« Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, pp. 

 209-232. 



