Position of the Axis of the Earth’s crust. 237 
kins’s extreme estimate to be true, yet elevations or depressions, 
uch as we know to-have taken place, of 8,000 or 10,000 feet, 
bear an appreciable ratio to the 800 or 1,000 miles which he as- 
signs as the thickness of the earth’s crust. 
t is, however, to be remarked that the extremely ingenious 
speculations of Mr. Hopkins are based on the phenomena of 
precession and nutation, and that if once the possibility of a 
change in theposition of the axis of rotation of the earth’s crust 
be admitted, it is not improbable that the value of some of the 
data upon which the calculations of these movements are found- 
ed may be affected. 
The supposition of the thickness of the crust being so great 
seems also not only entirely at variance with observed facts as 
to the increase of heat on descending beneath the surface of the 
earth, but to have been felt by Mr. Hopkins himself to offer 
such obstacles to any communication between the surface of the 
globe and its interior, that he has had recourse to an 
everywhere to be observed. Sir William Armstrong, on the as- 
sumption of the temperature of subterranean fusion being 3000 
Fahr., considers that the thickness of the film which separates 
from fiery ocean beneath would be about 84 miles. : 
Even assuming a thickness of 50 miles, so as to make still 
greater allowance for the increased difficulty of fusion unde 
avy pressure, the thickness of the crust would only form one- 
eightieth part of the radius of the earth; or if we represent the 
earth by a globe 13 feet in diameter, the crust would be one 
inch in thickness, while the difference between the polar and. 
€quatorial diameters would be half an inch. ieee 
In such a case, the elevation.or wearing away of continents 
such as are at present in existence, rising, as some of them do, 
nearly a quarter of a mile on an average above the mean sea- 
* Page, ‘ Advanced Text-book of Geology,’ p. 30. 
Am. Jour, Sct.—Srconp Sertes, Vou. XLIII, No. 128.—Maxcu, 1867. 
: 31 
