ERUPTIVE ROCKS. 299 



this is too uncertain a matter to enter into even so crude a calculation as 

 the present, and may therefore be neglected. The average specific gravity 

 of these beds may be assumed at 2.50, as the upper beds were probably 

 somewhat lighter than the average of those observed in this region. A 

 cubic foot would therefore weigh 155.8875 pounds, and 10,000 cubic feet 

 1,558,875 pounds, which is the theoretical pressure exerted by gravity on 

 each square foot of surface, and to raise this 1,500 feet would require a 

 force of 2,338,312,500 foot-pounds exerted on each square foot of surface. 



The above figures are to be considered rather as an indication of the 

 magnitude of the subterranean forces involved than an actual value of any 

 particular force, since the assumptions on which they are founded cannot be 

 mathematically proved. For instance, on the contraction theory of the 

 folding of the beds, the tangential strain to which they were already sub- 

 jected may have been sufficient to produce a tendency in the beds them- 

 selves to split apart, and thus in part have counteracted the theoretical 

 pressure exerted by gravity. 



Mathematical demonstrations, as applied to geological phenomena, are 

 at best of very doubtful value, owing to the impossibility of obtaining data 

 or measurements of an exactness that may be considered of mathematical 

 accuracy, and it often occurs that such demonstrations, which undoubtedly 

 display a high order of mathematical ability on the part of their author, are 

 comparatively worthless, or even misleading, owing to his assumption of a 

 premise which cannot be proved to be true. 



Source of intrusive force. What may have been the impelling force which 

 brought the fused material to its present position is evidently a purely specu- 

 lative question, and therefore hardly appropriate to be discussed here. What- 

 ever it may have been, it was undoubtedly of the same nature as that which 

 has caused flows upon the surface. Much ingenuity has been displayed by 

 theoretical geologists in discussing the source of volcanic energy, but in 

 the present stage of experimental or synthetic geology it is impossible to 

 find direct proofs for or against their views. The theory advanced by 

 Clarence King (op. cit.) is among the latest, and is deserving of considera- 

 tion because of his long and varied field experience. It may be stated in a 

 crude, brief way as follows : Starting with the assumption of a solid interior, 



