THE PAST AND FUTURE OF GEOLOGY. 191 



sarily follows that with the residual resistance reiluced to mere gravity, 

 a displacement, eudiug iu a state of equilibrium of the fractured strata, 

 would at ouce ensue, tbe amount of displacement being in proportion 

 to the severity of tbe strain. 



Tbe results of the foregoing conditions are in perfect accordance with 

 observation. The enormous crumpling and folding of the strata — tbe 

 vast upthrow of their disjointed edges — indicate the energy of the forces 

 which has been exerted, has been spent, and again accumulated. Of 

 these forces, it is as difficult for us to realize the intensity as it is to 

 fathom tbe immensity of space. These are among the questions for the 

 future. 



While thus refrigeration progressed, and the shell of the globe became 

 thicker, other causes came into operation to give it greater rigidity, and 

 so better fit it for the habitation of man. 



In the many discussions to which this question has given rise, it has 

 been too much assumed that the shell was of uniform or nearly uniform 

 thickness; the irregularities of the upper surface were apparent, but 

 those possible on the under surface have been scarcely sufficiently con- 

 sidered. There is, however, I think, reason to suppose, from some col- 

 lateral inquiries, tbat the under surface of the shell is ribbed and chan- 

 neled in a manner and on a scale materially to influence the operation 

 of that fluidity of the nucleus and mobility of the crust on which so 

 many able and elaborate calculations have been based. 



Let us take, on a continental area having a mean surface-temperature 

 of 55° Fahr., a point in the earth's crust throujih which an.v isotherm of 



depth passes— suppose it be that of 1,000°. This earth-isotherm will 

 possibly be found at a depth of between 50,000 and 100,000 feet.* The 

 isothermal plane must api)roximatively follow tbe contours of tbe sur- 

 face, and in mountain-districts, M may rise some 1,000 to 4,000 feet above 

 its other level. But when we come to seas, such as the Mediterranean, 

 the sea-bed has the mean tempeiature of the surface, (or n>or8 correctly 

 the mean temperature of tbe winter-months,) and tbe deptb of that bed 

 being from 0,000 to 8,000 feet, the earth's thermal plane of 1,000° is 

 thrown proportionally lower than on tbe adjacent land. 



*Tlu're is nsason to beliovo tlisit tlio niU) of iiicieiiso of tenipenituio of 1^ Fahr. for 

 every 50 to Gf) foot of depth, which obtains near the surface, is, owing to the increased 

 couductivo power of the rocks, tinfch less at greater depths. 



