On underground temperature; with observations on the conductivity of rocks; on the thermal effects of saturation and imbibition; and on a special source of heat in mountain ranges

Abstract

The opinions of physicists and geologists as to w hat may be the probable thickness of the crust of the Earth differ very materially. On the strength of its great rigidity and the absence of tides, physicists contend for a maximum thickness and the comparative solidity of the whole mass of the globe. On the evidence of volcanic action, the crumpling and folding of the strata in mountain ranges, its general flexibility down to the most recent geological times, and the rate of increase of temperature in descending beneath the surface, geologists contend for a crust of minimum thickness (although respecting the measure of this there is great difference of opinion) and a yielding substratum , as alone compatible with these phenomena. My intention here is not to enter upon the general question, but to lay before the Society the results of an inquiry on one section of it —namely, the rate of increase of temperature beneath the surface,—a subject equally affecting the argument on both sides. My attention was more specially directed to this subject in connexion with an inquiry on the cause of volcanic action, during which I found that the recorded observations gave so wide a choice in the selection of a mean rate for the increase of temperature, or as term ed by Professor Everett, the “thermometric gradient,” that very different values might be and were attached to it by different writers. I was thereby led to collect the scattered evidence bearing on the subject, with a view to see whether it were not possible to fix upon some more definite and less elastic rate.