THE ORIGIN AND DESCENT OF ROCKS. 399 



in the same mineral. A crystal may thus sometimes be seen, under the 

 microscope, to be made up of alternating layers of different silicates; 

 e.g., a microscopic layer of an aluminum-calcium silicate may be over- 

 lain by a microscopic layer of an aluminum-sodium silicate, and the 

 alternation may be repeated throughout the crystal, giving it a banded 

 structure. There is reason to beUeve that this is true in many cases where 

 the microscope fails to detect it, and that less symmetrical commin- 

 glings of silicates may take place. As such alternations or mixtures are 

 not governed by any known mathematical law, as is the case in chem- 

 ical compounds, there is no determinate limit to the number of com- 

 binations that may arise. As a matter of fact, new ones are still being 

 discovered in the progress of research, and the total number that may 

 ultimately be found can scarcely be prophesied. 



As a result of all this fertiUty of combination, the total number of 

 silicious minerals in igneous rocks is large. It is the function of the 

 mineralogist to treat of these minerals as such. The geologist deals 

 with them as constituents of the earth and as factors in its history. 

 Only a few of them are so abundant as to require special indi\ddual 

 notice in a general study of the earth. It may be remarked also that 

 only a few of them can be identified by simple inspection as they occur 

 in the rocks, partly because of the deHcacy of the distinctions between 

 many of them, and partly because of their minuteness and intricate 

 intermixture. The resources of the polarizing microscope are necessary 

 for safe determination in most cases. The student need not feel embar- 

 rassment or discouragement if he is often unable to recognize the con- 

 stituents of the intimately crystalline rocks. Their determination has 

 grown to be a profession by itself. 



The leading minerals of igneous rocks. — Fortunately for the sim- 

 plicity of geological study, a few minerals make up the great mass of 

 the igneous rocks. These few are quartz, the feldspathic minerals, the 

 ferromagnesian minerals, and the iron oxides. Quartz (siHca, SiOa) is 

 the free acid already mentioned. The feldspathic and ferromagnesian 

 minerals are the leading sihcates of the earth's crust, and vastly sur- 

 pass all others in abundance. The feldspathic group embraces minerals 

 formed by siHca in union with alumina, together with either potash, 

 soda, or lime, or two or more of these together. The ferromagnesian 

 group embraces minerals formed by the union of silica with iron, mag- 

 nesia, and Ume, together with more or less of the other basic oxides. 



