SEDIMENTARY AND METAMORPHIC ROCKS 91 
of the swamps, which are really destroyed lakes, there 
is often found a layer of powdery silica, so gritty that 
it is used for purposes of polishing. This is infusorial 
earth, being composed of the silicious skeletons of 
microscopic animals belonging to the group of Infu- 
Fig. 37. 
Limestone rock with fragments of coral, crinoids, and other fossils. 
soria. Sometimes this deposit is diatomaceous earth, 
so named because it contains large numbers of sili- 
cious shells of a plant belonging to the group of 
diatoms. 
Phosphate Rocks. In some places, as for instance 
near Charleston, $8. C., and in Florida, the bones of 
