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.

Summary.  The records of geology prove that since the earliest of their annals tremendous forces have been active in the earth.  In all the past, under pressures inconceivably great, molten rock has been driven upward into the rocks of the crust.  It has squeezed into fissures forming dikes; it has burrowed among the strata as intrusive sheets; it has melted the rocks away or lifted the overlying strata, filling the chambers which it has made with intrusive masses.  During all geological ages molten rock has found way to the surface, and volcanoes have darkened the sky with clouds of ashes and poured streams of glowing lava down their sides.  The older strata,—­the strata which have been most deeply buried,—­and especially those which have suffered most from folding and from fracture, show the largest amount of igneous intrusions.  The molten rock which has been driven from the earth’s interior to within the crust or to the surface during geologic time must be reckoned in millions of cubic miles.

The interior condition of the earth and causes of Vulcanism and deformation

The problems of volcanoes and of deformation are so closely connected with that of the earth’s interior that we may consider them together.  Few of these problems are solved, and we may only state some known facts and the probable conclusions which may be drawn as inferences from them.

The interior of the earth is hot.  Volcanoes prove that in many parts of the earth there exist within reach of the surface regions of such intense heat that the rock is in a molten condition.  Deep wells and mines show everywhere an increase in temperature below the surface shell affected by the heat of summer and the cold of winter,—­a shell in temperate latitudes sixty or seventy feet thick.  Thus in a boring more than a mile deep at Schladebach, Germany, the earth grows warmer at the rate of 1 degrees F. for every sixty-seven feet as we descend.  Taking the