296 
ME. GEOEGE H. DAEWIN ON THE INFLUENCE OF 
no means the same as its effective elevation or subsidence ; for erosion causes the effective 
to be far slower than the actual. And the actual upward or downward movement of 
an ocean-bed is different from the effective ; for the sea-water will flow off or in from 
the adjoining seas. The specific gravity of water is about one third of that of surface 
rock, and the local loss or gain of matter is the actual loss or gain of surface rock, less 
the mass of the sea-water admitted or displaced. Thus the effective downward or 
upward movement of a sea-bed is about § of the actual ; of this a more accurate esti- 
mate will be given presently. 
It is fortunately not important to track the series of changes through their course ; 
and in order to avoid the complication of doing so, the way seems to be to estimate the 
amount of transference of matter entailed in the conversion of a deep ocean into a con- 
tinent of the present mean height. 
Suppose, then, that an ocean area of 15,000 feet in depth were gradually elevated, 
and that the final result, notwithstanding erosion, were a continent of 1100 feet in 
height. Conceive a prism, the area of whose section is unity, running vertically 
upwards from what was initially the ocean-bed. Initially this prism contained 15,000 
feet of sea-water, and finally it contains 16,100 feet of rock; so that the local gain of 
matter, on this unit of area of the earth’s surface, is the difference between the masses 
of this prism, initially and finally. 
Now 1 ’02 is the specific gravity of sea-water, and 2‘75 that of surface rock; therefore 
the same local gain of matter, in a sealess globe, would be given by an elevation of 
16,100— iff of 15,000=10,436 feet. 
That is to say, 10,436 feet has been the effective elevation. 
I therefore adopt 10,000 feet as the effective elevation equivalent to the conversion 
of deep ocean into a continent ; and in the examples given hereafter, where I find the 
deflection of the pole for various forms and sizes of continent, I shall give the results of 
such an assumed conversion. 
19. Wide-spread Deformations of the Earth. 
It has hitherto been assumed that the elevation of land would not affect the sea-level ; 
but there can be no doubt but that elevations, such as those already spoken of, would 
do so to the extent of, say, a hundred feet. In so far, then, as this is the case, the eleva- 
tion would be masked from the eyes of geologists. But if the change of form were a 
gradual rising over a very wide area, the level surfaces would approximately follow the 
form of the rocky surface. For instance, the elliptical form of the equator carries the 
ocean level with it ; the amount of this ellipticity is such that the difference between 
the longest and shortest equatorial radii is 6378 feet*. So long, however, as these bulges 
remain equatorial they cannot affect the position of the principal axis, even should they 
vary in amount from time to time. But this kind of deformation, if not symmetrical 
* Thomson and Tait, Nat. Phil. p. 648. 
