VOLCANOES AND IGNEOUS INTRUSIONS 329 



Igneous Rocks 



Igneous rocks, as we have seen, have consolidated from a state of 

 fusion. The character of the rocks thus formed depends principally 

 (1) upon the chemical composition of the molten mass and (2) upon 

 the rapidity with which the magma cooled. Other conditions, such 

 as fluidity and pressure, are likewise important. 



Subdivisions Depending upon Chemical Composition. — Igneous 

 rocks which contain a large percentage of silica (65 per cent, or more) 

 are termed acid rocks, silica being an acid-forming oxide. Acid rocks 

 are usually light-colored when crystalline, and are lighter in weight 

 than basic rocks which contain much less silica (55 per cent, or less) 

 and a correspondingly larger amount of the bases, such as potash, 

 soda, lime, and magnesium. Basic rocks are usually dark-colored 

 and fuse at a lower temperature (p. 299) than acid rocks. They are 

 the common extrusive rocks and sometimes cover tens of thousands 

 of square miles of the earth's surface, and when weathered often 

 produce soil rich in plant food. 



Subdivisions Depending upon Texture. — The term texture as 

 applied to igneous rocks refers to their smaller features. When a 

 rock is described as being granitoid, or having a granular texture, the 

 reference is to one in which the crystals are distinct and are all of about 

 the same size. A rock with afelsitic texture is one composed of a mass 

 of very fine microscopic crystals. A rock is described as glassy when 

 it is made up largely or in part of glass in which no definite crystals 

 are to be seen. 



The rate of cooling appears to be the really important factor in deter- 

 mining the texture of igneous rocks, although other conditions have 

 considerable effect. The molten magma from which granitoid rocks 

 were crystallized was so deeply buried that the rate of cooling was 

 slow, thus giving an opportunity for the molecules of the same chem- 

 ical composition to gather to form large crystals and, consequently, 

 granitoid or granular rocks. Felsitic rocks 1 are the result of somewhat 

 more rapid cooling, and are found on the margins of great masses of 

 granitoid rocks which did not reach the surface, or in offshoots from 

 them in the form of dikes. Glassy rocks are those which cooled so 

 rapidly that the minerals had little opportunity to form. Rocks 

 with a glassy texture, consequently, occur chiefly in surface flows 

 and on the margins of dikes. 



1 Felsite is also often the product of the devitrification of glassy rocks. 



