512 
Van Hise—Earth Movements. 
which these quartzites were made be assumed to have been com¬ 
posed of spherical grains of uniform size, the amount of quartz 
required to fill the spaces would be .26 of that of the original 
grains of sand. 1 The minute interspaces are so well filled and 
the grains so firmly cemented that a quartzite breaks across the 
original particles rather than around them, giving a smooth 
conchoidal fracture like glass. But the change from sandstone 
to quartzite is a comparatively simple one. Far more profound 
changes have occurred on as great a scale in many countries. 
In some regions great igneous formations thousands of feet in 
thickness have been transformed into rocks which do not now 
contain a discoverable vestige of any original mineral. In other 
regions in which orogenic movements of the crust have occurred, 
the deeper seated rocks throughout have been completely re¬ 
crystallized. 
It is to be finally noted that as a result of the work of under¬ 
ground solutions the transferred material is on the average car¬ 
ried to a lower level. In the upper zone and especially that of 
weathering, material is dissolved, to be deposited deeper down 
in the earth, or to be brought to the streams for transportation 
to the sea. It is well understood that the underground waters 
may carry material upward, but it is certain that a much more 
than equivalent amount is carried downward. Thus the work 
of underground waters, like that of the surface agencies, is 
gravitative. 
The Power of Gravity .— Summarizing, we have seen that 
differential subsidence slowly but surely causes the continental 
masses to rise or fall with reference to the surface of the sea. 
As a result of the subsidence of great areas, smaller areas, 
such as the plateaus and mountain ranges and systems, may be 
elevated. The horizontal stresses, thickening the strata along the 
zones of plication and producing the mountain systems and pla¬ 
teaus, are but incident to the larger movements of subsidence. 
By vulcanism vast masses of magma are introduced into the outer 
part of the crust of the earth, or spread over its surface. The con¬ 
tinental areas, wherever they are above the sea, are being de- 
1 R. S, Woodward: Bull. Geol. Soc. Am., Vol. 1,1890, p. 220. 
