
z Part L. Sxcr. iii] UPHEAVAL AND DEPRESSION. 285 
5 yet been given, and that the subject still remains beset with many 
— difficulties. 
Mr. George H. Darwin, in one of his recent memoirs already 
cited (ante, p. 20), has suggested a possible determining cause of 
the larger features of the earth’s surface. Assuming for his theory a 
certain degree of viscosity in the earth, he points out that, under the 
combined influence of rotation and the moon’s attraction, the polar 
regions tend to outstrip the equator, and to acquire a consequent slow 
motion from west to east relatively to the equator. The amount of 
distortion produced by this screwing motion he finds to have been so 
slow, that 45,000,000 years ago, a point in lat. 30° would have been 
3’, and a point in lat. 60°, 141’ further west, with reference to the 
equator, than they are at present. This slight transference shows us, 
_he remarks, that the amount of distortion of the surface strata from 
this cause must be exceedingly minute. But it is conceivable that 
in earlier conditions of the planet this screwing action of the earth 
may have had some influence in determining the surface features of 
the planet. In a body not perfectly homogeneous it might originate 
wrinkles at the surface running perpendicular to the direction of 
ereatest pressure. “In the case of the earth the wrinkles would run 
north and south at the equator, and would bear away to the eastward 
in northerly and southerly latitudes, so that at the north pole the 
trend would be north-east, and at the south pole north-west. Also the 
intensity of the wrinkling force varies as the square of the cosine of 
the latitude, and is thus greatest at the equator and zero at the poles. 
Any wrinkle, when once formed, would have a tendency to turn 
slightly, so as to become more nearly east and west than it was when 
first made.” 
According to the theory, the highest elevations of the earth’s 
- surface should be equatorial, and should have a general north and 
south trend, while in the northern hemisphere the main direction of 
the masses of land should bend round towards north-east, and in the 
opposite hemisphere towards south-east. Mr. Darwin thinks that the 
general facts of terrestrial geography tend to corroborate his theo- 
retical views, though he admits that some are very unfavourable to 
them. In the discussion of such a theory, however, we must remember 
that the present mountain-chains on the earth’s surface are not abori- 
ginal, but arose at many successive and widely-separated epochs. 
Now it is quite certain that the younger mountain-chains (and these 
include the loftiest on the surface of the globe) arose, or at least 
received their chief upheaval, during the Tertiary periods—a com- 
paratively late date in geological history. Unless we are to enlarge 
enormously the limits of time which physicists are willing to concede 
for the evolution of the whole of that history, we can hardly suppose 
that the elevation of the great mountain-chains took place at an 
epoch at all approaching an antiquity of 45,000,000 years- Yet, 
according to Mr. Darwin’s showing, the superficial effects of internal 
distortion must have been exceedingly minute during the past 
