IMPORTANT ELEMENTS AND MINERALS 41 
(2) when exposed to the weather it begins to change 
and crumble. A set of complex reactions commences, 
_and in time the feldspar changes from the clear, hard, 
glassy mineral, to a dull, opaque substance, which can 
be scratched with a knife; and finally the change 
results in the production of a powdery, white clay, 
known as kaolin, from which some chinaware is 
made. 
If the chemist should analyze this clay, it would be 
found to vary considerably from the fresh mineral from 
which it was derived. Some of the original sodium 
or calcium, or potassium, has gone away, having 
entered into combination with other elements, produc- 
ing a soluble salt which could be removed by solution 
in water. It would also be found that a considerable 
percentage of water had been added to the clayey 
particles. So by chemical process, the form, texture, 
and the very composition have been changed. There- 
fore, when exposed to the weather, rocks which contain 
this mineral, decay and crumble, just as truly as a tree 
does after it dies and falls to the ground. 
We find feldspar, or its decayed products, nearly 
everywhere. Almost all the lavas that come from 
voleanoes, and the various forms of granitic rocks, 
bear one of the kinds of feldspar; and the clay of the 
soil, as well as of many fine-grained rocks, is in large 
part made of kaolin, while the waters of the globe 
