ON 'iiiii UKIGIN OP MOUNTAIN RANGES. 



163 



I am not here to-night to advocate any theory of my own. 

 I am inclined to believe that, after all that has been said 

 against it, the hypothesis of secular contraction has, mathe- 

 matics notwithstanding, some vitality in it yet. The view 

 that long-continned sedimentation gives rise to a line of 

 weakness (pai'tly through rise of the isogeothcrms) between 

 the continental area, strengthened perhaps by a depression 

 of the thermal planes throngh denudation, and the oceanic 

 area, strengthened perhaps by a depression of the isogeo- 

 therms through convection, and that along this line of 

 weakness a mountain range is ridged up through secular 

 contraction, still seems to mo, if not the most probable, at 

 any rate the least improbable hypotliesis. 



I do not question for a moment the accurate working of 

 the mathematical mill which has produced Sir William 

 Thomson's and Mr. Osmond Fisher's results. But I think 

 the data may need revision. And the accuracy of the result 

 depends entirely upon the validity of the data. Sir William 

 Thomson's results are based, it seems, upon the cooling of 

 (1) a solid globe, (2) by conduction. But accepting for the 

 nonce M.v. Fisher's intorstratum of molten rock, which we 

 may presume has been gradually lessening in amount by 

 gradual solidification : has suiEcient account been taken of 

 the consequent contraction on solidificiiJiion to granite or 

 other such rock, estimated by M. Delcsso at from three to 

 seven per cent. ? has sufficient account been taken of the 

 removal from the underlayors of vast amounts of rock 

 through the instrumentality of volcanic action? and has 

 Rufficiont account been taken of the large amount of heat- 

 cnorgy removed by the underground circulation of water, 

 and by the vast volumes of steam formed during volcanic 

 eruptions ? The annual discharge of the Bath spring alone 

 is more than sixty-five million gallons at a temperature of 



