514 
Van Hise—Earth Movements. 
maintained. The greater the depth of the mass moved, the 
greater the thickness of the excess or deficiency of the mass 
which may be maintained. The harder the rocks, the greater 
the thickness of the excess or deficiency which may be main¬ 
tained. The higher the temperature, the less the thickness of 
the excess or deficieny which may be maintained. The greater 
the water content, the less the thickness of the excess or defi¬ 
ciency which may be maintained. The more rapid the deforma¬ 
tion, the greater the thickness of the excess or deficiency which 
may be maintained. All of these statements are sufficiently 
self-explanatory except the last. Where rocks are deformed rap¬ 
idly, the elastic limit is higher than where deformed slowly. 
Also during uplift of the continental areas, erosion works in the 
opposite direction. The more rapid the uplift, the more effective 
is erosion. As the elevation becomes great, erosion becomes 
very rapid, and uplift slow, because the elastic limit of the rock 
is neared. Therefore the actual limit of uplift is less than that 
which could be temporarily sustained by the strength of the 
rocks. 
Conclusion .—I hope this paper has made it clear that in the 
part of the earth we know there is movement everywhere; that 
the forces are constantly at work re-shaping, re-making the 
world. Even within the rocks themselves, porous or apparently 
ogy, Chicago). The central thesis of this paper is that the modulus of 
compression of rocks varies under pressure in various ways under different 
conditions. As illustrative of his applications, erosion may be taken. 
Because of the shifting of material by epigene agencies to the sea shore, 
there is additional weight, and consequent compression, which results in 
subsidence. The denudation of the land removes load, which results in 
expansion, and therefore elevation. If Major Powell’s hypothesis be ac¬ 
cepted, it seems to me to accord with the law above advocated, that is, 
the sum-total of the movements are gravitative. For the compression is 
compensated for by expansion, and the lateral transfer is accompanied by 
downward movement. 
I would make gravity explain the phenomena of deformation “ of the 
earth’s crust as the law of gravity explains the constitution of the celestial 
systems.” (Powell.) Indeed, the geological history of the geoid is the 
part of the astronomical history which has been studied in detail. The 
geoid continues to be controlled in its deformation through geological 
time as it has in pre-geological time, and as are other worlds and the suns, 
— by the force of gravity* 
