GENERAL DEDUCTIONS 221 



most readily decomposed are those containing protoxides of iron 

 and manganese, or lime, and the first indication of decomposi- 

 tion is signalled by a ferruginous discoloration and the appear- 

 ance of ealcite. The evidence bearing upon the relative dura- 

 bility of the various minerals constituting rocks is, however, 

 quite conflicting and unsatisfactory. Doubtless much depends 

 on local conditions. 



Dana observed^ that in the decomposition of the granitic 

 rocks of the Chilean coast the feldspars yielded first, becoming 

 white and opaque and of a friable earthy appearance. But it 

 should be noted that there is nothing in Professor Dana's de- 

 scription to show that this change may not have been a purely 

 physical one, and due to the splitting up of the feldspars along 

 cleavage lines. Fournet, from a study of the processes of kao- 

 linization, was led to state^ that hornblende yields less readily 

 to decomposing forces than does feldspar, when the two are 

 associated in the same rock. Becker, however, in studying 

 deep-seated decomposition in the Comstock Lode of Nevada, 

 arrived at a precisely opposite conclusion, the feldspars as a 

 whole offering more resistance than the augite, hornblende, or 

 mica. Lindgren noted^ that the decomposition of the California 

 grano-diorite manifested two distinct phases. The first, due to 

 the decomposition of the feldspar grains alone resulted in re- 

 ducing the rock to a soft, crumbling mass. In the second stage, 

 the biotite and hornblende also are decomposed, the biotite being 

 the more refractory. 



The present writer has described^ thick sheets of augite por- 

 phyrite in Gallatin County, Montana, in which the feldspathie 

 disintegration has gone so far that the mass falls away to a 

 coarse sand, from which still perfectly outlined crystals of coal- 

 black augites may be gleaned in profusion. This last is, 

 however, a semi-arid region, and the process thus far one of 

 disintegration more than decomposition. In a moist, or perhaps 

 in any climate, minerals consisting essentially of silicates of 

 alumina and magnesia are less liable to decomposition than 

 those containing considerable proportions of iron protoxides or 

 of lime. This for the reason that the first-named are scarcely 



^Eeport Wilkes's Exploring Expedition, Geology, p. 578. 



2 Ann. de CMmie et de Physique, Vol. LV, 1833, p. 240. 



« Seventeentli Ann. Rep. U. S. Geol. Survey, Part II, 1895-96, p. 39. 



*Btill. U. S. Geol. Survey, Ko. 110, 1894. 



