Orogenic Movements. 
491 
themselves, place the deep-seated materials below them under 
great stresses, and they flow from beneath them toward the in¬ 
tervening arches, where the stresses are less than normal, and 
thus helps to support them. 
Looked afc in another way, an entire plateau may be consid¬ 
ered as a great arch of such magnitude that the strength of the 
rocks is insufficient to support the mass. At various zones the 
arch collapses. At such places the rocks are plicated and thick¬ 
ened and these zones are the traversing mountain ranges. This 
point of view does not substantially alter the principles of up¬ 
lift as above given. 
It may, perhaps, be doubted whether the explanation offered 
is adequate to account for the existence of the more extensive 
plateaus. But, if the causes assigned are not adequate, I am 
unable to suggest supplementary causes. However such plat¬ 
eaus may have been produced, it appears highly probable from 
gravity measurements, made by Putnam, 1 and discussed by him 
and Gilbert, that the plateaus of western America have an 
amount of material in excess of that of the low lands, correspond¬ 
ing in most cases to their elevation. Therefore it appears cer¬ 
tain that whatever the causes of these uplifts the rigidity of the 
rocks is sufficient to sustain them for a long time, and it may 
be plausibly argued that if the rocks are strong enough to sus¬ 
tain the excess of material, they were strong enough to produce 
the uplift as suggested. Whether or not Thibet has an excess 
of material corresponding to its elevation is unknown. This 
■can only be ascertained by gravity observations. Until such 
observations determine the excess of material in this region, 
we cannot tell to what extent this plateau is sustained by the 
rigidity of the rocks. 
Returning to the mountain systems, we find they are not lo¬ 
cated by accident. The rule appears to be, as stated by Hall, 2 
that where great masses of sediments are being piled up as a 
1 Results of a transcontinental series of gravity measurements, by G. R. 
Putnam; and Notes on the gravity determinations reported by G. R. Put¬ 
nam, by G. K. Gilbert: Phil. Soc. Wash., Vol. XIII, 1895, pp. 31-76. 
2 Contributions to the geological history of the American continent, by 
James Hall: Proc. Am. Assoc. Adv. Sci., Vol. 31, 1882, p. 55. 
