170 Analyses of Books . [March, 
Sir W. Thomson’s assumption of the total solidity of the 
earth’s interior rests upon grounds, none of them decisive, and 
two of which he has in effect given up. Supposing that the 
earth is, as a whole, extremely rigid, it does not follow that it is 
wholly rigid from centre to circumference. If further, the eleva- 
tions on the earth’s surface are due to the contraction of a hot 
solid globe, cooling by conduction to and radiation from the 
surface, the average height of the elevations above the datum 
level can be calculated to a few feet. If we suppose that the 
temperature near the surface increases at the rate of i° F. per 51 
feet, and that the rocks have been solidified at the temperature 
of 7000° F. (an extreme supposition), the average height of the 
elevations from the era of solidification down to the present time 
would be only between 800 to 900 feet. If we take 4000° F. 
(the temperature of melting slag) as the more probable point of 
solidification, the average elevation will be below 200 feet. But 
even the highest of these assumptions is wide of the truth. The 
average height of the elevation of the earth’s surface above 
datum level is at the lowest estimate 9500 feet. Consequently, 
Sir W. Thomson’s assumption of a wholly solid earth, fails as 
leading to results glaringly at issue with faCts, and his calcula- 
tions based upon such assumption may be safely pronounced 
null and void. 
The following remark, though direCtly connected with the 
present argument, is also important in reference to the constancy 
of the distribution of land and water: — “The islands of New 
Zealand occupy a central position in the aqueous hemisphere, and 
yet they contain a series of deposits chronologically analogous to 
those of the northern hemisphere ; whence it follows that they 
must often have been submerged during periods, when there 
existed, in what is now an extended ocean-land, surfaces from 
which the materials of their rocks must have been derived. The 
sinking of river-plains and estuaries, a phenomenon of general 
occurrence, is also significant. The author observes, what is 
required to explain such faCts is a liquid or at least a plastic 
substratum for the earth’s crust to rest upon. In connection 
with the supposed permanent position of continents and oceans 
from the earliest times, Mr. Fisher doubts whether the doCtrine is 
fully proved.* It is suggested that a large portion of the ocean 
now above the earth’s crust may have once been below it, and 
thus we gain a novel conception of a sense in which the foun- 
tains of the abyss may once have been broken up. 
Passing over much important matter, we come to the author’s 
examination of Mr. Mallet’s explanation of subterranean heat, as 
manifested in volcanic aCtion, in hot springs, and in the high 
temperature of deep mines. That heat he considers as “ pro- 
* See Quart. Journal Geolog. Soc., xxxvi., p. 351, and Geol. Mag., Decade 
II., vol. vi., p. 385. 
