540 E. W. Hilgard on Mallets Theory of Vulcanicity. 
sized fragments intermingled (such as, rather than powder, 
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 co-effi- 
cients resulting from his rock-crushing experiments, at all 
necessary to the establishment of the postulates of his theory. 
Taking for granted the correctness of Hirn’s theorem, “ that 
the heat evolved in the crushing of rigid bodies is the equiva- 
lent of the work performed,” Mallet’s experiments on the con- 
traction of fused rock in cooling, and his estimates of the 
arcing: must be supposed to originate, this condition must 
e closely approached ; especially in the early times of the vol- 
aare” 
lee 
