1 882.] Analyses of Books. 169 
authority, to fix a maximum limit for the time which has elapsed 
from the solidification of the earths’ surface down to the present 
day. The results reached, though based upon an imposing array 
of figures and formulas, vary from 33 to 300 million years, or in 
the proportion of 1 : 10! Surely such discrepancies might have 
suggested reasonable doubts as to the value of the whole investi- 
gation. Yet, on the faith of calculations so discordant a kind 
of flank attack has been made upon the teachings of geology, 
and to a certain extent upon the doctrine of Organic Evolution. 
Thus Prof. Tait says : — “ Geology, in framing its conclusions, is 
compelled to take into account the teachings of other sciences. 
If we feel disposed to give an indefinite amount of time to the 
evolution of cosmos out of chaos, the student of heat and 
mechanics comes in and asserts upon the authority of his branch 
of science, that a limit must be put to the time available for 
bringing about the present condition of things : he will grant 
100 to 300 millions of years as the extreme allowance of time; if 
geologists cannot be content with this allowance, so much the 
worse for the geologists, for more they cannot have.” Now it 
may be possible and is indeed probable, that the mutations in 
organic life have not proceeded at one uniform rate. It may 
also be possible, that the high tides due to the former closer 
proximity of the moon (See “Journal of Science,” 1882, p. 43) 
may have effected denudations at a much more rapid rate than 
is now going on, and consequently that geological changes may 
have been effected in comparatively short times. Still the 
majority, both of geologists and biologists feel bound to reject 
Sir W. Thomson’s results, as based upon arbitrary and unproved 
assumptions. Among these suppositions are : — That the earth, 
though once fluid is now a solid, cooling by conduction ; that 
the temperature of the earth’s crust increases as we penetrate 
deeper, but at a gradually decreasing rate ; which, at about 5000 
feet below the surface, becomes nil. That the molten matter 
ejected by volcanic action is “ derived ” from the transformed 
work of crushing the rocks of the earth’s crust, owing to the con- 
traction of its interior through long-sustained cooling. 
These assumptions, Mr. Fisher traverses very ably, and, in 
our opinion, additional weight is due to his arguments, because 
they are brought forward without any regard to the requirements 
either of geologists or biologists. In his first chapter, he 
discusses the experiments which seem to indicate that the rise 
of temperature on boring deeper into the earth proceeds at a 
gradually decreasing rate, and points out several circumstances 
by which the results are rendered doubtful. His conclusion is, 
that “ we know nothing by observation, respecting the law of 
increase of temperature within the earth, beyond the facts, that 
near the surface the average rate of increase is i° F. for between 
50 and 60 feet of descent, and that the temperature at greater 
but unknown depths is very high, sufficient to fuse what, from 
that very cause, we term igneous rocks.” 
VOL. IV. (third series). n 
