338 Transactions. — Oeohgy, 



whatever retarding effect its atmosphere may have exercised 

 upon the coohug of the outer crust, that coohng was compara- 

 tively rapid, although the straining effects which I assume to 

 have resulted from the causes referred to were still powerful 

 enough, in Tertiary times, to result in the elevation of nearly 

 if not all the great mountain chains now existing upon it. 

 Wliether the forces in question are still equal to bringing about 

 changes in the surface similar to those which are revealed to us 

 by the investigations of geologists as having occurred since the 

 commencement of the Eocene period, can only be determined in 

 the far distant future, although I am inclined to doubt it. 



The straining referred to has, however, certainly not ceased, 

 and will not cease until the thickness of the earth's rigid crust 

 has become sufliciently great to prevent further solidification of 

 the molten interior matter. The diminution which has appa- 

 rently taken place in the intensity of volcanic action since the 

 close of the Miocene period, seems to indicate the approach of 

 such a condition of things, and that time, when it does arrive, 

 will certainly be the commencement of the period in which the 

 earth will attain its ultimate surface conditions. 



Art. XLIII. — On the Cause of Volcanic Action. 



By J. Hardcastle. 



[Read before the Hawke's Bay Philosuphical Society, I'dth September, 1886.] 



Abstract. 



The first section of this paper reviews at length the arguments 

 iu favour of the dynamical theory for the origin of volcanic 

 force, and the opinions accepted by the author may be sum- 

 marized as follows : — 



The conversion into heat of the work expended on the 

 crushing and other internal rearrangement of rocks, (generally 

 as subordinate })lienomeuain mountain elevation,) by horizontal 

 pressures produced iu the crust of the earth by its sinking upon 

 a retreating nucleus, under the action of gravity, is the eliicient 

 source of volcanic heat of all degrees of intensity. The pres- 

 sures, and the effect of th«ir conversion into heat, may be 

 roughly calculated. A specimen calculation shows the pressures 

 required to elevate a mountain range 120 miles wide, 8i miles 

 high above its supporting base, and from a crust 5G miles thick, 

 must be 340 tons per square inch, the work of which, converted 

 into heat, would raise the temperature of any mass of silica 

 within which it acted by about 4,200^ Fah., and other rocks iu 



