\%% Mr. J. Croll on the Physical Cause of the 



regarding their cause and origin. The great diversity and ex- 

 treme character of those changes, as indicated by the remains of 

 ancient flora and fauna, arc such as to render it difficult to find 

 any possible cause adequate for the effect. 



The point regarding which the greatest difficulty has been 

 felt, is in accounting for the extreme cold of the glacial epoch, 

 and the warm and tropical character of the carboniferous. 



Before entering on the consideration of a cause which appears 

 in a great measure to have been overlooked by geologists, we 

 shall briefly refer to a few of the more prominent theories which 

 have been advanced to account for those changes. 



The warm character of the climate during the Silurian, Car- 

 boniferous and other periods of the Palaeozoic age, was at one 

 time generally referred by geologists to the influence of the 

 earth's internal heat. But it has been proved by Professor 

 William Thomson* that the general climate of our globe could 

 not have been sensibly affected by internal heat at any time 

 more than 10,000 years after the commencement of the solidi- 

 fication of the surface. And Mr. Hopkins has concludedf that the 

 present effect of internal heat is only about ^yth of a degree on 

 the mean superficial temperature. Professor W. Thomson, from 

 calculations based upon more correct data, has lately found that it 

 onlyamounts to aboutyjth of adegree J. Professor Phillips, how- 

 ever, is still of opinion that the warm climates of ancient epochs 

 may have been due to the influence of internal heat. He does not 

 question the correctness of the calculations made by Thomson, 

 Fourier, Poisson, and others, but thinks that they have over- 

 looked the fact that the condition of the earth's atmosphere, as 

 regards its power of conducting heat, might have been different 

 in former ages from what it is at present. u The state of the 

 atmospheric mantle/' he says, " which envelopes the terraqueous 

 globe, mitigates solar heat and stellar radiation, and, like the 

 clothing of a steam cylinder, prevents excessive waste of the 

 warmth treasured within "§. It is quite true, ^as Professor 

 Phillips suggests, that a diminution in the conductivity or in 

 the diathermancy of the earth's atmosphere, and an increase in 

 its height, would increase the influence of internal heat on the 

 climate. But when we reflect that under the present condition 

 of the atmosphere the internal heat could not even sensibly 

 affect the climate after the short period of 10,000 years from 

 the commencement of solidification of the earth's surface, it 

 appears very improbable that our atmosphere could have ever 



* Phil. Mag. for January 1863. 



f Journal of the Geological Society, vol. viii. 



X Proceedings of the Royal Society of Edinburgh for March 21, 1864. 



§ Life on the Earth, p. 163. 



