CONDITIONS NEVER UNIFORM. 189 



on the surface, we are still without a shadow of evidence 

 that this interior heat has exercised the least perceptible 

 effect on the climatology of the globe since the deposition 

 of the fossiliferous strata.* On the contrary, all that we 

 know of the nature and thickness of pre-Cambrian sedi- 

 ments, and all that we have learned of the Cambrian rocks 

 themselves, preclude the supposition of such an influence 

 beyond the most infinitesimal degree, and compel us to 

 believe that the physical conditions of life have been much 

 the same throughout every period of its existence. It is 

 true that during the Carboniferous period, the Oolite, and 

 the earlier Tertiaries, certain latitudes of the northern hemi- 

 sphere appear to have enjoyed a more genial climate than 

 they do now ; but the explanation of this we are to seek in 

 the varying distributions of sea and land, the existence of 

 warmer oceanic currents and other geographical conditions, 

 rather than in any perceptible influence derived from the 

 earth's interior, ^ay, as we have warmer and colder regions 

 in space, in virtue of the earth's relations to the solar system, 

 so w r e are inclined to believe we have had warmer and 

 colder periods in time, in virtue of some great but unknown 

 cosmical law. The existence of the Glacial or boulder epoch 

 over the greater portion of the northern hemisphere (say 

 up to the 40th or 42d parallel of latitude) is now ad- 

 mitted on all hands ; and as we cannot entertain the idea of 



* According to Fourier, Hopkins, and other physicists, the internal 

 heat of the globe, which increases at the rate of 1 degree for every 60 

 feet of depth, does not at present affect the mean superficial tempera- 

 ture more than l-20th of a degree ; and to have had any sensible effect 

 on external climates say to the exent of 10 degrees this interior heat 

 must have been two hundred times its present amount. At that rate 

 the melting point of lavas would have been reached at a depth of 580 

 feet, instead of 116,000 feet as presently estimated, and all the deeper 

 seated strata must have been fused or rendered crystalline a condition 

 in which they do not occur even to the depth of 30,000 feet, as many of 

 the Cambrian and Silurian slates are merely hardened and cleaved, but 

 in no degree metamorphie. 



