280 Transactions. — Geology. 



It cannot be doubted that beyond a mile or two beneath the 

 surface the rocks are so well supported on all sides that 

 enormous force must be required to crush them. Under such 

 circumstances there can, of course, be no crushing into frag- 

 ments. The conditions preclude any separation of the parts 

 or particles to form interstices. But any change of shape, 

 any forcible deformation, producing movement of the com- 

 ponent particles amongst each other, would be equivalent 

 to crushing to dust, so far as development of heat is concerned ; 

 possibly more than equivalent, since in crushing to dust power 

 is absorbed in overcoming cohesion, and, I apprehend, does not 

 reappear as heat ; whereas it is conceivable that in deformation 

 merely less of the force is so absorbed. 



If a pressure of ten times 15 tons per inch is sufficient to fuse 

 average rock, and, say, 300 or even 500 to fuse harder rocks, 

 such pressures are available for the purpose. They are mere 

 fractions of the stupendous pressures that the collapse of the 

 crust would give rise to, if the rocks were rigid enough to call 

 them forth. The possible crust pressures exceed 5,000 tons per 

 inch, which gives a wide margin of crushing force over any pos- 

 sible resistance. The conversion of the work of the extn 

 pressure into heat would give solar temperatures to a consider- 

 able quantity of rock ; so that the crushing theory easily 

 accounts for terrestrial volcanoes, giving as wide a margin of 

 temperature, almost, as of crushing power. 



This development of Mr. Mallet's theory completely disposes 

 of Professor Hutton's dust-heap objection ; and the remainder of 

 his " difficulties," those relating to the earthquake observations, 

 instead of being difficulties, are proofs of the correctness of 

 the crushing theory. There had been occasional earthquakes 

 in the locality for some months before the eruption, indicating (for 

 that is what an earthquake does indicate) that some portion of 

 the crust was in motion, yielding to contractile pressures in 

 some direction or other. Those crust movements may have 

 occurred, but by no means necessarily so, in the exact region 

 where the volcanic outburst afterwards originated, and they maj 

 have had much or little to do with bringing about the catas- 

 trophe. It is impossible for any massive movement in the crust 

 to occur without heating in some degree the rocks which arc 

 crushingly affected by the movement, whether by friction of 

 opposed surfaces, or deformation of larger or smaller masses. 

 There may, however, be movements producing violent earth- 

 quakes which do not result in the heating of rocks up to fusing 

 point, or to a temperature capable of vaporising water under the 

 pressure due to the depth. On the oilier band, a series of small 

 movements, indicated by slight earthquakes, successively attack- 

 ing and deforming the same mass .it rock, will successively 

 increase its temperature, even up to fusing point, if the attack 



