864 STUDIES FOR STUDENTS 



apparent that hydration is an important factor, the amount of 

 water increasing rapidly, as decomposition advances. There is, 

 moreover, among the siliceous crystalline rocks in every case a 

 loss in silica, a greater proportional loss in lime, magnesia, and 

 the alkalies, and a proportional increase in the amounts of alum- 

 ina and sometimes of iron oxides, though the apparent gain may 

 in some cases be due to the change in condition from ferrous to 

 ferric oxide. As a whole, however, there is a distinct loss of 

 materials, though the residuary product may actually contain a 

 larger percentage of certain constituents than did the rock from 

 which it was derived. 



According to Bischof and as shown in our own work the sili- 

 cates in any rock that are most readily decomposed are, as a 

 rule, those containing protoxides of iron and manganese, or 

 lime, and the first indication of decomposition is signaled by a 

 ferruginous discoloration and the appearance of calcite. 



Fournet, from a study of the processes of kaolinization, was 

 led to state ^ that the hornblende yields less readily to decompos- 

 ing forces than does feldspar, when the two are associated in the 

 same rock. Becker, however, in studying deep-seated decom- 

 position 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. 



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

 phyrite in Gallatin county, Montana, in which the feldspathic 

 disintegration has gone on 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, how- 

 ever, in a semi-arid region, and the process thus far more one of 

 disintegration than decomposition. 



In any climate, minerals consisting chiefly of silicates of 

 alumina and magnesia are less liable to decomposition than those 

 containing iron protoxides, or lime carbonates, for the reason 

 that the first named are not easily affected by carbonic acid. 



'Ann. de Cliimie et de Ph3'sique, Vol. LV, 1833, p. 240. 

 ^ Bull. U. S. Geol. Survey, No. no, 1894. 



