Mallet's Theory of Vulcanicity. 51 



interior liquid, unless either such folding occurs beneath the 

 general level of the liquid, or the latter is locally confined, 

 or the movement is so (comparatively) brusque or cataclysmal, 

 that viscosity would prevent the lateral or downward escape of 

 the liquid rock. In the case of the Pacific eruptions the evi- 

 dence of steady static outflow and regular upbuilding is espe- 

 cially cogent ; and, as LeConte remarks, it has been slow work, 

 as indeed is usually or universally the case with mountain- 

 building*. 



The assumption of locally limited fire seas with a solid globe 

 as made by Danaf in conformity with Hopkins's views, would 

 remove the difficulty if the crust could be assumed as contract- 

 ing on the whole independently of the portions over fire seas. 

 But when we come to discuss the application in detail of this 

 intrinsically improbable hypothesis, we find the required ex- 

 tent and localities of these fire seas to be such that we can 

 hardly imagine them to be effectually separated from each 

 other ; in other words, we approach very near to a condition of 

 general undercrust fluidity up to late geological periods J. It 

 then becomes a question of minor importance whether there is 

 a central nucleus solidified by pressure, or whether all within 

 the crust is actually liquid. 



The inherent improbability of the depression of a geosyn- 

 clinal trough to a level so low as to allow the liquid rock 

 to rise into it, as it were, is too great to render its discussion 

 necessary. 



Indeed it seems almost impossible to imagine a mechanism 

 explaining satisfactorily fissure-eruptions such as those of the 

 Pacific coast, on the basis of a slowly contracting solid crust 

 with a rapidly contracting liquid layer or nucleus beneath. A 

 more satisfactory explanation seems possible if, in accordance 

 with Mallet's suggestion and the intrinsic probabilities of the 



* When LeConte says (loc. cit. p. 179) that the outsqueezing of the 

 liquid has been caused by " enormous horizontal pressure, determined by 

 the interior contraction of the whole earth/' and then (p. 180) that, 

 " whether by uplifting or upbuilding, the actual increase of height would 

 be precisely the same, being determined by the amount of lateral crush- 

 ing," he seems to think of crust-contraction upon a nucleus too large for 

 it, rather than of Mallet's " freely descending " crust. Or, if he considers 

 the fused rock the result of motion transformed, it is difficult to see on 

 what ground a simple "uplifting" could be considered the precise mecha- 

 nical equivalent of an upbuilding by eruption of liquid rock. In either 

 case the lifting done would be the same ; but what of the enormous heat 

 of fusion ? 



t "On some of the Results of the Earth's Contraction," Silliman'a 

 American Journal, August 1873, p. 105. 



t Ibid. Julv 1873, p. 7 et seqq. 



E 2 



