ORIGIN OF THE SOIL 21 



forces. The rocks from which all the other rocks are sup- 

 posed to have been derived are called igneous rocks. They 

 are hard and compact, and include the granites, diorites, ba- 

 salts, and lavas. Usually they are found deep in the earth 

 and are buried under not only many feet of mantle rock but 

 of other bed rocks as well. Rocks derived from the igneous 



Photograph by II. L. Hollister Land Co. 



FIG. 8. Glacier-covered slope in the Rocky Mountains 

 Note the banks of rock fragments which have been carried down by the ice 



rocks are called sedimentary, or aqueous, rocks because they are 

 regarded as having first been laid down in the bottom of shal- 

 low lakes or seas as beds of mud, sand, or gravel, and later 

 compacted into rock. Limestones, sandstones, and shales are 

 some of the better-known sedimentary rocks. Some of the 

 sedimentary rocks have been subjected to great heat and 

 pressure since their formation, thus altering their structure. 

 Such rocks are called metamorphic rocks. Slates, marbles, 



