4 Constitution of Rocks. 



By far the larger proportion of the surface rocks 

 of the world have been formed by the agency of water, 

 chiefly as a fluid, but partly as ice. Such rocks are 

 made of sediments, and these sediments have been, and 

 still are, chiefly the result of the action of atmospheric 

 agencies, aided by chemical solutions, and of gravita- 

 tion, aided by moving water. But by what special pro- 

 cesses were they formed ? 



Air and water, but especially the latter, act both 

 chemically and mechanically on the crust of the earth. 

 Many minerals in rocks, such as felspars, hornblendic 

 minerals, mica, &c., are composed of silicates of alum- 

 ina and soda, potash, lime and magnesia. These are 

 often associated with free silica. This is especially the 

 case with some igneous rocks ; and many of the strati- 

 fied rocks consist in great part of substances of the same 

 nature variously intermixed. Others consist of carbon- 

 ate and sulphate of lime, &c., more or less pure. 01 

 these, the carbonate of lime rocks, or common lime- 

 stones, by far predominate ; and they are sometimes 

 nearly pure, forming immense areas of country, and 

 sometimes mechanically intermingling, in every per- 

 centage, with other substances. All rain as it falls 

 absorbs part of the carbonic acid in the air ; and the 

 water percolating through the rocks unites with and 

 carries away in solution portions of the soda, potash, 

 lime, or magnesia that enter into the composition of 

 the minerals in rocks, and this promotes their disinte- 

 gration. They crumble, and are in a condition to be 

 borne to lower levels, and finally to the sea, by the me- 

 chanical agency of running water, or partly in solution. 



Frost is also a powerful disintegrator. Water per- 

 colates into hollows, joints, and cracks ; it freezes and 

 expands, and thus helps to rend and break up the rocky 



