470 
Van Hise—Earth Movements. 
implied by the illustration of the ship. The rocks have a very 
considerable rigidity, and in order that a readjustment shall 
begin, the difference in vertical stresses must be sufficient to 
overcome the rigidity of the rocks. The stress-difference re¬ 
quired is probably far short of the elastic limit of rocks as de¬ 
termined by experiment under ordinary conditions of pressure 
and temperature. According to Gilbert, 1 the excess of gravity 
in the Rocky Mountains is measured by some 2200 feet of 
material. This is steadily tending to lower this area. Whether 
it is sufficient to cause any movement is uncertain. Wher¬ 
ever there is an excess of material sufficient to cause sub¬ 
sidence, it is probable that the movement is exceedingly slow, 
for nowhere is the weight of the excess known to approach the 
crushing strength of the stronger rocks. The excess required 
to give a weight sufficient to crush such rocks would be a thick¬ 
ness of rock material of about 20,000 feet. 2 In areas where 
the excess is sufficient to produce subsidence, the process would 
doubtless go on with decreasing speed, because of the steady 
decrease of the stresses. Movement resulting from excess of 
pressure will not continue until perfect equilibrium is reached, 
because it will cease the moment the stresses are unable to 
overcome the rigidity of the rocks. But as explained later 
(See p. 472), equilibrium may possibly result from loading of 
one area, combined with denudation of another. 
As a result of the erosive action of wind, water, and ice, 
the continents are constantly being degraded, and the higher 
they stand above the surface of the seas, other things being 
equal, the more rapidly does the process go on. If, with a suf¬ 
ficient number of locomotives, all the freight cars in the United 
States were continuously at work carrying to the sea the earth 
of our continent, the average haul being taken at 1,500 miles, 
and no time being taken for loading or emptying, there would 
be carried to the sea but a little more than twice the amount 
of material contributed to the Gulf of Mexico by the Missis- 
1 New light of isostacy, by G. K. Gilbert: Journ. of Geol., Vol. Ill, 
1895, p. 332. 
2 Principles of North American pre-Cambrian geology, by C. R. Van 
Hise: Sixteenth Ann. Rept. U. S. Geol. Survey, Pt. I, 1896, p. 592. 
