46 Mr. E. W. Hilgard on some points in 



subject for the proof of the efficiency of this agency. But we 

 find that the maximum of temperature resulting from the crush- 

 ing to powder* of the hardest rock is something over 217° 

 Fahr. This, then, represents the maximum increment of tem- 

 perature that can be rendered efficient toward the fusing of 

 rocks by the crushing process uifder the most favourable cir- 

 cumstances, viz. upon the supposition that it takes place in- 

 stantaneously, or under such circumstances that the heat can- 

 not be conducted away, and, further, that the resistance of the 

 rock has not been materially diminished by the downward in- 

 crease of hypogeal temperature. At the most moderate depths 

 at which volcanic phenomena can be supposed to originate, the 

 last-mentioned factor must exert a very considerable influence, 

 reducing materially the available heat-increment. Hence the 

 numerical results of Mallet's laborious experiments on rock- 

 crushing, however interesting and useful as affording a definite 

 measure of the thermal effects producible by this means, yet 

 fail to carry conviction as to the efficacy of this particular modus 

 operandi in reducing large masses of solid rock to fusion, unless 

 essentially supplemented by friction, not so much of rock walls 

 against each other, but more probably by the heat produced 

 within more or less comminuted detrital or igneoplastic masses 

 by violent pressure and deformation. 



It may be doubtful what would be the physical and thermal 

 effect of enormously great pressures upon rock powder such as 

 was produced in Mallet's experiments ; but it would seem that 

 if made to yield, the frictional effect must produce very high 

 temperatures. A fortiori, solid detrital masses of variously 

 sized fragments intermingled (such as, rather than powder, 

 would be likely to result from steady pressure), yielding rapidly 

 under great pressures, might, under the combined influence of 

 friction and rock- crushing, well be supposed to reach the tempe- 

 rature of fusion, which a simple crushing of a solid mass by 

 pressure would have failed to produce. Mallet mentions the 

 probable influence of friction, and of the squeezing of igneo- 

 plastic masses, but does net attach to these agencies such im- 

 portance as they seem to me to deserve. 



Of the complex thermal effects of the movements of detrital 

 masses under great pressure, Mallet's figures of course offer no 

 measure whatsoever ; nor is this, or even the thermal coefficients 

 resulting from his rock-crushing experiments, at all necessary 

 to the establishment of the postulates of his theory. 



* Mallet does not go into the consideration of the physical nature of this 

 "powder," and of the thermal and other differences likely to result from 

 its production under pressures enormously greater than those employed 

 by him. 



