414 



referred to is generally termed " kaolinization ; " it is important, however, 

 to bear in mind that all cases of kaolinization are not due to simple 

 surface decomposition. In speaking of the origin of the constituents of 

 the water of the Great Geyser, we have already referred to a process 

 of subterranean kaolinization, and we shall have again to refer to such 

 a process in dealing with the different varieties of deep-seated meta- 

 morphism. Next in importance to the felspars are the ferro-magnesian 

 minerals the pyroxenes, hornblendes, olivines and dark micas. These 

 are also liable to decomposition by ordinary surface-agencies. 



Lime, which enters largely into the composition of many of these 

 minerals, is readily removed in the form of carbonate. Magnesia is 

 far less readily removed and often gives rise to the formation of 

 hydrous silicates, such as serpentine, chlorite, &c. Iron may be either 

 removed or deposited, according as the decomposition is effected under 

 the influence of reducing or oxidizing conditions. Epidote frequently 

 arises in connection with the decomposition of the ferro-magnesian 

 constituents, or as a result of the reaction between the solutions thus 

 formed and the constituents of the felspars. The ultimate products of 

 the decomposition of the non-aluminous minerals are carbonates, ferric 

 oxide and quartz. Iron ores become fully oxidized under the influence 

 of atmospheric conditions. Quartz and muscovite are comparatively 

 stable under these conditions, and are, therefore, removed by mechanical 

 rather than chemical means. The minute accessory constituents, such 

 as zircon, tourmaline, rutile and spinelle are also comparatively stable 

 under atmospheric conditions, and owing to their small size are but 

 little acted upon by the mechanical agencies by which the insoluble 

 constituents of igneous rocks are removed and deposited. We accordingly 

 find them in almost all deposits of fine sand and clay. 



It appears, therefore, that igneous rocks may be decomposed and 

 disintegrated by what we have called the normal surface-agencies. In 

 many cases, however, the rocks have been more or less altered before 

 being exposed at the surface by denudation ; so that, in considering 

 the causes of decomposition it is impossible to separate completely 

 the effects of deep-seated agencies from those which operate directly 

 on the surface. The plutonic rocks especially are often, as it were, 

 prepared by the deep-seated agents of change for disintegration under 

 surface conditions. 



The solid matters which thermal springs bring to the surface 

 furnish, as we have already seen, direct evidence of subterranean 

 changes brought about by the percolation of water through the rock. 

 As to the precise nature of these changes we are generally ignorant, 

 but we may call attention to one or two instances of wide-spread 

 alteration in igneous rocks which are almost certainly due to the 

 cause referred to. 



The Comstock Lode occurs in a region of highly-altered igneous 

 rocks. It is difficult to define the area of alteration with anything 

 like precision in consequence of the absence of well-defined boundaries, 



