They consolidated also under pressure. They are never scoriaceous, for the steam with which they were charged was not allowed to expand and distend them with steam blebs. In the rocks of the larger intrusive masses one may see with a powerful microscope exceedingly minute cavities, to be counted by many millions to the cubic inch, in which the gaseous water which the mass contained was held imprisoned under the immense pressure of the overlying rocks.
Naturally these characteristics are best developed in the intrusives which cooled most slowly, i.e. in the deepest-seated and largest masses; while in those which cooled more rapidly, as in dikes and sheets, we find gradations approaching the texture of surface flows.
Varieties of the intrusive rocks. We will now describe a few of the varieties of rocks of deep-seated intrusions. All are even grained, consisting of a mass of crystalline grains formed during one continuous stage of solidification, and no porphyritic crystals appear as in lavas.
Granite, as we have learned already, is composed of three minerals,—quartz, feldspar, and mica. According to the color of the feldspar the rock may be red, or pink, or gray. Hornblende—a black or dark green mineral, an iron-magnesian silicate, about as hard as feldspar—is sometimes found as a fourth constituent, and the rock is then known as hornblendic granite. Granite is an acidic rock corresponding to rhyolite in chemical composition. We may believe that the same molten mass which supplies this acidic lava in surface flows solidifies as granite deep below ground in the volcanic reservoir.
Syenite, composed of feldspar and mica, has consolidated from a less siliceous mixture than has granite.
Diorite, still less siliceous, is composed of hornblende and feldspar,—the latter mineral being of different variety from the feldspar of granite and syenite.
Gabbro, a typical basic rock, corresponds to basalt in chemical composition. It is a dark, heavy, coarsely crystalline aggregate of feldspar and augite (a dark mineral allied to hornblende). It often contains magnetite (the magnetic black oxide of iron) and olivine (a greenish magnesian silicate).
In the northern states all these types, and many others also of the vast number of varieties of intrusive rocks, can be found among the rocks of the drift brought from the areas of igneous rock in Canada and the states of our northern border.