36 
AMERICAN GEOLOGY. 
latter, by its transporting power, and by the aid of frosts in 
breaking up the strata and disintegrating the exposed surfaces. 
The entire operation may be summed up in two processes, one 
of which degrades the more elevated parts of the earth’s sur¬ 
face, and the other fills up the depressed portions. Fire or heat 
operates in four ways: 1. In the elevation of areas by the 
application of expansive forces beneath the earth’s crust, by 
which it is raised up in mass. 2. By the transference of 
fused matter from the interior to the surface, and which it 
overflows, and thereby makes an additional thickness to the 
visible strata. The addition being transferred from the interior 
to the exterior, may be in the form of melted matter, semifluid 
matter, or in the form of mud, or in pulverulent matter, in the 
condition of ashes and semifused mass ejected from the craters 
of volcanoes. 3. In consequence of the loss of matter thus 
thrown out from the interior, areas of subsidence are formed, 
and the superficial strata are engulfed suddenly, or else slowly 
subside and sink below their former levels. 4. Areas are ele¬ 
vated or depressed by the simple expansion of strata by heat 
and their contraction by cold. 
The force generated by heat is proportionate to its intensity. 
It pervades, in a limited degree, the zones of rock immediately 
beneath the earth’s surface. This is proved by its increase 
downward from the limit of solar influence, which is a point of 
no variation for the year. The ratio of increase for this country 
is one degree of Fahrenheit for every fifty-five or sixty feet, and 
for Europe one degree for every forty-five or fifty feet. These 
facts point to a source of heat in the earth’s interior. This 
view is supported by the overflow of immense quantities of 
incandescent and melted matter from volcanic vents. Like all 
other bodies, rocks are expanded by heat and contracted by 
cold, and these changes in volume are connected both with 
changes of level in the earth’s crust and in its disruption, or 
the forcible separation of continuous strata, and the formation 
of intervening fissures. In the simple expansion of rocks 
by heat, and their subsequent contraction by cold, we have an 
