CHANGES IN THE STRATIFIED ROCKS 275 
naturally more pronounced. Then even silica may 
be made to serve as a cement, and a sandstone be 
transformed to a dense, solid quartz mass. Indeed, 
by the action of hot water, metamorphism may begin, 
and a series of very complex changes commence, by 
means of which the original rock is finally altered 
beyond recognition. Generally the cementing of rocks 
is the result of several agencies, among which water 
is the most essential. 
Several substances are common as rock cements, 
particularly carbonate of lime, the salts of iron and 
silica. Nearly all the limestones, many of the clay 
rocks, and some of the sandstones are cemented by 
carbonate of lime. Iron as a cement is most common 
in sandstones, and less frequently binds together the 
fragments of clay rocks. The same is true of silica. 
Frequently two or three of these substances unite to 
cement the same rock. Because of this ready action 
of water, nearly all the sedimentary strata of the 
land are changed from their original loose, incoherent 
condition, to solid rock. 
Concretions. — In the geyser basins of the Yellowstone Park, 
some of the silica brought to the surface in the hot water, is 
gathered together into balls which are growing in size by con- 
stant additions. These more or less spherical accumulations of 
silica are known as geyserites (Fig. 43). 
On many shores where the water carries much calcite in solu- 
tion, a part of this is deposited so as to form tiny spheres of 
