76 
IDDINGS. 
Certain combinations of minerals, however, are character¬ 
istic of rocks with certain chemical characters, and are those 
generally observed; but exceptional combinations occur. 
Thus, of the basic rocks, basalt is almost universally com¬ 
posed of augite, olivine, basic plagioclase, and magnetite or 
ilmenite, with no orthoclase, hornblende, biotite, or quartz; 
but instances are known where each of these minerals occurs 
as a primary constituent. Gabbro is ordinarily composed of 
diallage or augite, basic plagioclase, an orthorhombic pyrox¬ 
ene, titaniferous iron oxide, and often olivine, but in excep¬ 
tional cases it may contain hornblende, biotite, orthoclase, 
and quartz. 
With the acid rocks the reverse order obtains. They are, 
in the great majority of cases, composed of quartz, orthoclase, 
alkali-plagioclase, biotite, and hornblende, without the min¬ 
erals characteristic of the basic rocks, but in a comparatively 
small number of cases the latter minerals also occur. 
The various kinds of rocks, therefore, differ in the relative 
amounts of the essential minerals, which may belong to a 
few of the rock-making species or may represent a greater 
number of them. 
Character of the Minerals as Crystals .—The history of the 
crystallization of each rock is written more or less completely 
upon the mineral crystals composing it, and may be read 
from their form, their internal or material character, and 
also from their relations to one another. 
(1.) The outward form or shape of a crystal indicates 
something of the conditions undei* which it crystallized. In 
glassy rocks the outlines of thin sections of crystals are 
almost always straight-edged, and correspond to definite 
crystal planes or faces. This is especially true of the larger 
crystals. It indicates that the minerals crystallized freely 
from a fluid or molten magma. Microscopic crystals often 
exhibit irregular forms, which are not directly referable to 
crystallographic planes. They assume fantastic shapes in 
