CLAY AND ITS MODE OF OCCURRENCE. 5 



This process alone, if kept up, may reduce the rock to a mass 

 of small angular fragments. 



The rain water, however, acts in another way. It not only 

 carries oxygen into< the pores of the rock, but also acids in 

 solution, the latter having been gathered in part from the decay- 

 ing vegetable matter in the soil and in part from the air. The 

 result of this is that the oxygen and the acids attack many of 

 the mineral grains of the rock and change them into 1 other com- 

 pounds. Some of these are soluble and can be carried off by the 

 water circulating through the mass, but others are insoluble and 

 are left behind. It will thus be seen that one effect of this action 

 is to withdraw certain elements from the rock, and, the structure 

 of the minerals as well as the rock being destroyed, it crumbles 

 down to a clayey mass. 



The three minerals mentioned as being commonly present in 

 granite are not equally affected, however, by the weathering 

 agents. Thus the quartz grains are but slightly attacked by the 

 soil waters, while the feldspar loses its lustre and changes slowly 

 to a white, powdery mass, which is usually composed entirely of 

 grains of kaolinite. The mica, if whitish in color, remains un- 

 attacked for a long time, and the glistening scales of it are often 

 visible in many clays. If the mica is dark colored, due to iron 

 in its composition, it rusts rapidly and the iron oxide, thus set 

 free, may permeate the entire mass of clay and color it brilliantly. 



If now a granite, which is composed chiefly of feldspar, 

 decays under weathering action, the rock will be converted into 

 a clayey mass, with quartz and mica scattered through it. Re- 

 membering that the weathering began at the surface and has 

 been going on there for a longer period than in deeper portions 

 of the rock, we should expect to find on digging downward from 

 the surface, a) a layer of fully formed clay, b) below this a 

 poorly defined zone containing clay and some partially decom- 

 posed rock fragments, c) a third zone, with some clay and many 

 rock fragments, and d) below this the nearly solid rock. (Fig. 

 1.) In other words, there is a gradual transition from the fully 

 formed clay at the surface into the parent rock beneath. The 

 only exception to this is found in clays formed from limestone, 

 where the passage from clay to rock is sudden. The reason for 



