EARTH'S CRUST — WASHINGTON. 273 



referred to as " nrineralizers." These gases also play an important 

 part in the formation of many ore bodies. 



The magma on solidification generally forms a mixture of minerals, 

 substances of definite chemical composition, and physical characters, 

 just as a solution of salts in water (such as sea water), forms a mix- 

 ture of crystals of salts and ice on freezing. The exception to this 

 is when the cooling of the magma takes place too rapidly for com- 

 plete or (as with the obsidians) any crystallization, in which case 

 the rock is composed partly or wholly of glass. Such glassy rocks are 

 found only as surface flows. 



MINERAL CONSTITUENTS OF ROCKS. 



It is a very important and striking fact that, although about 1,000 

 different minerals are known, yet the number of the different kinds 

 that compose by far the great majority of igneous rocks — certainly 

 over 99 per cent by weight of these — is very small. Indeed, the really 

 important and essential igneous rock-forming minerals number only 

 about a dozen. 



These essential minerals are quartz, silicon dioxide ; the feldspars, 

 silicates of alumina and potash, soda, or lime, including the potassic 

 orthoclase, the sodic albite, and the calcic anorthite, with isomorphous 

 mixtures of these; the pyroxenes, metasilicates of calcium, magne- 

 sium, and iron, with aluminum or sodium in some cases; the am- 

 phiboles, in chemical composition much like the pyroxenes, but differ- 

 ing in crystal form and otherwise; the micas, alumino-silicates, for 

 the most part the potassic muscovite or the potassium-iron-magne- 

 sium biotite, both containing hydroxy! ; the olivines, orthosilicates of 

 iron and magnesium; nephelite, an orthosilicate of sodium and 

 aluminum; leucite, a metasilicate of potassium and aluminum; mag- 

 netite, ferroso-ferric oxide, often containing titanium; and apatite, 

 h phosphate of calcium, containing a little fluorine or chlorine. Mag- 

 netite and apatite are present in almost all rocks, but seldom in more 

 than almost negligible amounts. 



Other minerals are not infrequently met with in certain types of 

 igneous rocks, such as the silicates sodalite, hauyne, melilite, zircon, 

 and garnet, and the oxides tridymite (a second form of silica), ilme- 

 nite, chromite, spinel, corundum, and rutile. But, considering igne- 

 ous rocks from the standpoint of a study of the whole crust of the 

 earth, these are practically negligible. Igneous rocks, then, in gen- 

 eral, and looked at in the broadest way, are constituted almost wholly 

 of a very few silicates of aluminum, iron, calcium, magnesium, 

 sodium, potassium, and hydroxy!, with or without quartz (that is, 

 excess of silica), with small amounts of a phosphate and of iron 

 42803°— 22 18 



