64 ELEMENTARY GEOLOGY 
solidified and crystallme. From top to bottom the 
material is the same; but the rocks which result will 
differ greatly in appearance. At the surface may be 
glass and a fine-grained lava (Fig. 19 and Plate 2); 
but deep down in the earth a granite rock is formed 
(Fig. 20). Geologists classify igneous rocks, and give 
them names, according to the variations in texture, as 
is illustrated in the table on p. 66. 
Variation in Composition. — But volcanoes in differ- 
ent parts of the earth, reach down to reservoirs which 
furnish entirely different kinds of lava. That from the 
Mexican volcanoes, for instance, is light in color, while 
the lava of the Hawaiian Islands is dense black. When 
subjected to a chemical analysis, these are found to 
differ in the kind and proportion of elements. The 
lighter lavas have more of the acid-forming elements 
(of which silicon is an example), and these are said to 
be acid rocks; the darker, or basic lavas contain a 
smaller percentage of acid-forming elements, and more 
of the basic (potassium, magnesium, iron, etc.). With 
this difference there is a variation in the minerals; 
for with much silicon, for instance, quartz can be 
formed in abundance, while it must be nearly if not 
quite absent from the basic lavas, which contain a 
small percentage of silicon. 
Classification. — So igneous rocks are classified and 
given names (1) according to their texture, which 
