THE PAST AND FUTURE OF GEOLOGY. 193 



the principle of a solid crust of great thickness, has proi)ose(l a theory 

 to account for the continued ejection of molten matter from depths not 

 far beneath the surface, and acting independently of any common source 

 of lava supply, by the conversion of the energy resulting from crushing 

 into heat along given lines of intense pressure. 



It seems, however, to me that the uniform character appertaining to 

 volcanic eruptions over the whole world, the traveling of earthquake 

 movements, the flexibility yet evinced in movements of the crust, and 

 the magnitude of the later geological changes, precludes the acceptance) 

 of the conditions suggested by these distinguished physicists, and leads 

 me to seek for other causes to account for the i)resent stable condition 

 of the earth. 



The cause which suggests itself to me is the intense cold of the glacial 

 period through which the earth has so recently passed, and which has, 

 as it were, anticipated or forestalled the refrigeration which, in orduiary 

 course, would have taken a longer time to effect, and so would have been 

 prolonged into some subsequent period. At present, the annual varia- 

 tion of temperature in these latitudes extends to a depth of about 30 feet; 

 the maximum heat of summer being felt underground by the end of 

 November, and the maximum cold of winter b^^ the beginning of June, at a 

 depth of 20 feet. But supposing the cold of winter at dei)ths not to alter- 

 nate with and be influenced by summer heat, then the abstraction of 

 heat would continue to a depth in proportion to the length of time during 

 which the cold at the surface was maintained ; and such must have been 

 the conditions over a large portion of the northern hemisphere (and I 

 believe of the southern contemporaneously) during the glacial period. 

 For as permanent ice and snow then extended down to these latitudes, 

 the summer sun would not sensibly affect surfaces so covered, and the 

 abstraction of heat must have proceeded uninterru[)tedly. To what 

 depth the effect may have extended has not yet been investigated, but 

 that it must have been very considerable is evident from the depth to 

 which the annual variations are now felt. Consequently, with a uniform 

 permanent temperature of 32° or lower at the surtace, and considering 

 the long duration of the glacial i)eriod, we may form some conception of 

 how i'ar beneath the surface the extreme cold must have extended ; 

 even now, in parts of Siberia, the ground is permanently frozen to a 

 depth of 300 to 400 feet. Then the surface temperature in tliese lati- 

 tudes, instead of commencing as now with a mean ot r)()o, and attaining 

 a degree, say of 70^, at a depth of 1,000 feet, commenced with a tem- 

 perature of 32° F. or less,. and the isothermal of 70° must have been 



melting tcmperatiuc, uiid aftcrwiird cooling as a solid, the amount of cnnuplings and 

 contortions of the surface which could be produced by its subsequent refrigeration 

 would be very much smaller than sufficient to account for the existing inequalities of 

 the earth's surface ; and hence he concluded that such has not been the mode in wiiich 

 the earth luis attained its present state, but that a crust commenced to form before the 

 interior became solid. (See also Mr. Fisher's paper "On the Elevation of Mountain- 

 Chains " iu Trans. Cambridge Phil. Soc. for 18G9, vol. xi, part iii.) 



S. Mis. 11a 1 a 



