MATERIALS OF THE EARTH 27 



The two leading micas are the iron-magnesia mica, biotite, or 

 black mica, and the potash mica, muscovite, or white mica, the 

 familiar "isinglass" of the stove-door. Chemically, muscovite 

 should go with the potash feldspars, but it is distinguished from 

 them by its crystalline habit and physical properties. Chem- 

 ically, too, biotite should go with the pyroxenes and amphiboles, 

 which it closely resembles except in the form and properties 

 of its crystals. 



Two iron oxides, magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ), are 

 widely disseminated in igneous rocks, but they are far less abun- 

 dant than the silicates. They constitute the free bases already 

 mentioned. 



Summary of salient facts. The salient facts are, therefore, 

 (1) that out of the 70-odd chemical elements in the earth, eight 

 form the chief part of it; (2) that one of these elements uniting 

 with the rest forms nine leading oxides; (3) that one of these 

 oxides acts as an acid and the rest as bases; (4) that by their com- 

 bination they form a series of silicates of which a few are easily 

 chief; (5) that these silicates crystallize into a multitude of minerals 

 of which again a few are chief; and (6) that these minerals are 

 aggregated in various ways to form rocks. Possessed of these lead- 

 ing ideas, we are prepared to turn to the consideration of some 

 of the conditions under which these combinations take place in 

 the formation of rocks from liquid magmas. 



Classification of Igneous Rocks 



Several features are involved in the current classification of igneous rocks 

 Some of these features have been noted already, but they may be recapitu- 

 lated here. All fragmental igneous rocks are pyroclastic, and pyroclastic 

 rocks may be tuffs, agglomerates, etc. (p. 21). Rock formed by the solidi- 

 fication of lava without the development of crystals, is obsidian, if not porous. 

 If porous (hardened rock-froth), they are pumice, scoriaceous glass, etc. 

 If the rock is largely glass, but partly of small crystals, it is sometimes called 

 pitchstone, because its freshly fractured surface looks like pitch or resin. 

 When scoriaceous rock has its cavities filled by minerals deposited from 

 solution, the rock becomes an amygdaloid. Porphyry has already been 

 defined (p. 23). All these names are based on texture, rather than on min- 

 eralogical or chemical composition. 



Most igneous rocks are wholly crystalline, and are classified on the basis 



