98 ELEMENTARY GEOLOGY 
and in the sea: more than one-half of the crust of the 
earth that is exposed to view, is made of sedimentary 
Fie. 43. 
Geyserites ; large odlitic balls from geyser region of Yellowstone Park. Reduced 
about one-half. 
strata. Therefore every one should be familiar with 
these important rocks." 
METAMORPHIC Rocks 
Nature of the Process. —The nature and origin of 
these rocks cannot be clearly stated until some know- 
ledge of the conditions under which they are formed 
is obtained: this part of the subject is taken up on 
1 No study of these or other rocks is adequate without actual examination 
and close study of specimens. These, like the minerals and igneous rocks, 
can be obtained at slight cost from the mineral dealers. (See p. 23.) 
