ORIGIN AND FORMATION OF SOILS. 117 
These substances are again compounds of phosphate of 
lime, 8 CaO P,O,, with chloride of calcium, Ca Cl,, or 
fluoride of calcium, Ca F,, respectively. 
§ 3. 
ROCKS—THEIR KINDS AND CHARACTERS. 
The Rocks which form the solid (unbroken) mass of 
the earth are sometimes formed from a single mineral, but 
usually contain several minerals in a state of more or less 
intimate mixture. 
We shall briefly notice those rocks which have the 
greatest agricultural importance, on account of their com- 
mon and wide-spread occurrence, and shall regard them 
principally from the point of view of their chemical com- 
position, since this is chiefly the clue to their agricultural 
significance. Some consideration of the origin of rocks, 
as well as of their structure, will also be of service. 
Igneous Rocks.—A share of the rocks accessible to 
our observation are plainly of igueous origin, i. e., their 
existing form is the one they assumed on covling down 
from a state of fusion by heat. Such are the lavas that 
flow from volcanic craters. 
Sedimentary Rocks.—Another share of the rocks are 
of aqueous origin, i. e., their materials have been deposit- 
ed from water in the form of mud, sand, or gravel, the 
loose sediment having been afterwards cemented and con- 
solidated to rock. The rocks of aqueous origin are also 
termed sedimentary rocks. 
Metamorphic Rocks,—AStill another share of the rocks 
have resulted from the alteration of aqueous sediments or 
sedimentary rocks by the effect of heat. Without suffer- 
ing fusion, the original materials have been more or less 
converted into new combinations or new forms. Thus 
limestone has been converted into statuary marble, and 
