I96 IGNEOUS ROCKS 



The acidic rocks are so called because they are rich in silica 

 (which is an acid), but they have only small quantities of lime, 

 magnesia, and iron ; hence they are very infusible, of low specific 

 gravity, and generally of light colours. The basic rocks, thus 

 named because of the predominance of the bases, have much 

 smaller percentages of silica and higher ones of lime, magnesia, 

 and iron ; the latter substances act as fluxes, making the basic 

 rocks much more fusible, as well as giving them a higher specific 

 gravity and darker colour. The distinction between acidic and 

 basic rocks cannot be very sharply drawn, because the two kinds 

 are connected by every variety of intermediate gradation. The 

 same is true, however, of all the divisions given in the table, 

 which is apt to produce a false impression of sharply distinguished 

 groups of rocks, such as do not occur in nature. 



As a general rule, the glassy and porphyritic textures charac- 

 terize those rocks which have solidified at the surface of the 

 ground, or not very far below it, while the granitoid types have 

 cooled slowly and at great depths ; but there are exceptions to 

 both statements. Between the glassy and porphyritic textures at 

 one end of the series and the granitoid at the other, comes the 

 felsitic, which represents an- intermediate rate of cooling and 

 intermediate depths within the earth as the place of solidification. 



The division of the igneous rocks into families is made prima- 

 rily in accordance with the mineralogical composition, with sub- 

 divisions according to texture. This method gives us five principal 

 groups. 



I. The Granite Family 



The molten magma, which on solidification gives rise to the rocks 

 of this group, is very rich in silica (65 to 75%) and has from 10 

 to 15% of alumina; the quantity of alkalies (Na and K) is rela- 

 tively large (6 to 8%), and there are small amounts of iron 

 oxides (2 to 4%), magnesia (1 to 2%), and lime (1 to 4%). 

 In the process of consolidation the principal minerals formed are 

 orthoclase and quartz, with smaller amounts of oligoclase, iron 

 oxide, and of the ferro-magnesian minerals, biotite or hornblende. 



