THE COMPOSITION OF THE EARTH 



23 



and is then also called bog iron ore. It gives a yellowish brown 

 streak. 



Of the minerals described above, quartz, feldspar, hornblende, 

 augite, and mica make up the bulk of most igneous rocks. Few 

 igneous rocks have a very large amount of any other mineral. 



IGNEOUS ROCKS 



As noted above, igneous rocks are the product of the con- 

 solidation of lavas. The character of an igneous rock is 

 determined by (1) the chemical composition of the parent 

 lava, and (2) the conditions under which that lava solidified. 



FIG. 7. Granite, about % natural size. The light parts represent crystals 

 of two kinds of minerals, and the dark spots represent crystals of other 

 minerals. (Photograph by Baker.) 



The kinds and proportions of chemical elements in a lava deter- 

 mine the kinds of minerals and their relative abundance in the 

 rocks derived from it. Under different conditions of solidi- 

 fication, lava of a given chemical composition produces rocks 

 of very different appearance. The mineral grains, for ex- 



