CHAPTER IV 



THE ROCKS OF THE EARTH'S SURFACE SHELL 



The processes by which rocks are formed. Rocks may be 

 formed in any one of several ways. When a portion of the molten 

 lithosphere, so-called magma, cools and consolidates, the product 

 is igneous rock. Either igneous or other rock may become dis- 

 integrated at the earth's surface, and after more or less extended 

 travel, either in the air, in water, or in ice, be laid down as a sedi- 

 ment. Such sediments, whether cemented into a coherent mass 

 or not, are described as sedimentary or clastic rocks. If the fluid 

 from which they were deposited was the atmosphere, they are 

 known as subaerial or eolian sediments ; but if water, they are 

 known as subaqueous deposits. Still another class are ice-deposited 

 and are known as glacial deposits. 



But, as we have learned, rocks may undergo transformations 

 through mineral alteration, in which case they are known as 



metamorphic rocks. 

 When these changes 

 consist chiefly in the 

 production of more 

 soluble minerals at 

 the surface, accom- 

 panied by thorough 

 disintegration, due 

 to the direct attack 

 of the atmosphere, 

 the resulting rocks 

 are called residual 

 rocks. 



The marks of ori- 

 gin. Each of the 



three great classes of rocks, the igneous, sedimentary, and meta- 

 morphic, is characterized by both coarser and finer structures, in 

 the examination of which they may be identified. The igneous 



30 



Fio. 16. Laminated structure of sedimentary rock, 

 Western Kansas (after a photograph by E. S. 

 Tucker). 



