ORIGIX AXn FOUMATION OF SOILS. Ill* 



Tliese substances are again compounds of phosphate of 

 lime, 3 CaO P^O^, with chloride of calcium, Ca Cl„ or 

 fluoride of calcium, Ca Fl^, respectively. 



§3. 



ROCKS-THEIU 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 brielly notice those rocks which have the 

 greatest agricultural importance, on account of their com- 

 mon and wide-spread occurrence, and shall regard them 

 pi'incipally from the point of view of their chemical com,' 

 position, since tliis 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 igneous origin, i. e., their 

 existing form is the one they assumed on cooling 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. — Still another share of the rocks 

 have resulted from the alteration of aqueous sediments or 

 sedimentary rocks by the efiect of heat. Without suffer- 

 ing fusion, the original materials have been more or less 

 converted into new combinations or uev forms. Thus 

 limestone has been converted into sta<"uary marble, and 



