THE ORIGIN AND DESCENT OF ROCKS. 401 



boles, which it closely resembles except in its crystalline properties. 

 Details respecting the micas may be found in the reference Hst, p. 464. 



Two iron oxides, magnetite (FcgOJ and hematite (FCjOg) are 

 widely disseminated in igneous rocks. They constitute the free 

 bases already mentioned. 



Summary of salient facts. — The saHent facts are, therefore, (1) that 

 out of the seventy-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',a;il acid 

 and the rest as bases; (4) that by their combination they form a series 

 of siUcates of which a few are easily chief; (5) that these sihcates crys- 

 taUize 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 leading ideas, we are prepared to turn to the con- 

 sideration of some of the conditions under which these combinations 

 take place in the formation of rocks from molten magmas. 



THE NATURE OF MOLTEN MAGMAS. 



We easily fall into the habit of thinking of molten rock as we think 

 of a molten metal, merely as a substance which has passed from the soHd 

 to the hquid condition because of high temperature. With the return 

 of low temperature a molten metal returns to the sohd state usually in 

 the same molecular condition which it possessed before. The point of 

 fusion and the point of solidification are the same and are rigidly fixed. 

 If this were true of the constituents of a rock, a definite order for the 

 solidification of the several minerals might be anticipated. As a matter 

 of fact, the order is not the same under all conditions, and, what is 

 especially significant, the order is far from being that in which the con- 

 stituents would fuse or would sohdify separately. For instance, in a 

 granite composed chiefly of quartz, feldspar, and mica, the quartz is 

 often the last to take form, although it is more infusible than the feld- 

 spar or the mica. This and other phenomena show^ that a molten magma 

 is not to be viewed simply as a fused substance, but rather as a solution 

 of one silicate in another, or as a solution of several silicates in one 

 another mutually. The high temperature is to be regarded merely as 

 a condition prerequisite to solution, or as the condition of fusion of 

 some one constituent which then dissolves the others. If crystals 



