32 Poulett-Scrope — On Mallet's Theory of Volcanic Energy. 



■which it proceeds consist of such unascertainable quantities. For 

 example, take Mr. Mallet's experiments on the crushing force of 

 various rocks. These will, no doubt, be valuable to architects and 

 engineers ; but, being made upon small cubical blocks in a dry state, 

 at the temperature of 57°, and subject on four of their sides to no 

 other resistance than that of the atmosphere, are wholly inconclusive 

 as to the effect of pressure on similar rocks miles under ground, 

 permeated with water, at temperatures probably far exceeding 

 1000°, and in contact on all sides with resisting media at least as 

 unyielding as themselves. Moreover, the effect of pressure upon 

 mineral matter at exceedingly high temperatures in raising or 

 lowering their fusing points is, from the difficulty of conducting the 

 experiments, as yet, I believe, extremely obscure. And this addi- 

 tional source of uncertainty is not touched upon in Mr. Mallet's 

 investigations. 



Mr. Mallet's further experiments on the contraction of rock matter 

 in passing from a state of liquid fusion to one of solidity by loss of 

 heat are useful, as reducing the probably exaggerated estimates of 

 Bischoff, who set down the loss of volume in granite on cooling at 

 nearly one-fourth, while Mr. Mallet places that of basic slag-matter 

 at about six per cent., and of acid silicates at considerably less. 

 But Mr. Mallet himself adds that " as applied to our globe it is 

 highly probable that any inferences that may be drawn from these 

 experiments must be subject to the changes in volume that may 

 have arisen as the mass cooled from changes in its molecular 

 arrangement, such as that from the vitreous to the crystalline con- 

 dition, data for which are unknoion" (§ 173). 



All then that Mr. Mallet is entitled to say is that, on the hypothesis 

 of a gradually cooling globe, some portion of the heat which is sensibly 

 perceived to escape from its interior by conduction through the 

 superficial rocks, and convection by means of hot springs and vol- 

 canic eruptions, may possibly be generated by the compression or 

 crushing of the crust as it follows the shrinking nucleus. Any 

 attempt to measure the amount of heat produced in this manner, and 

 to compare it with that which directly proceeds from the trans- 

 mission outwardly of the original heat of the interior, is, I think, 

 in the present state of our knowledge of that interior, and of the 

 forces acting there, unprofitable, or at all events premature. Indeed, 

 Mr. Mallet himself admits (§ 181) that " we are obliged by the 

 phenomena of hypogeal temperature to conclude that by far the 

 largest proportion of the heat annually lost reaches the surface from a 

 cooling nucleus." Where then, I ask, is there any evidence that the 

 remaining fractional portion of this heat is really produced by com- 

 pression of subsiding rock, and is "the true source of volcanic 

 energy," as Mr. Mallet maintains it to be ? 



Mr. Mallet's theory fails, I think, to account for the fact that 

 volcanic eruptions are almost wholly coixfined to certain lines or 

 bands traversing the earth's surface, and apparently indicating the 

 existence through long geologic ages of great rents in the solid crust, 

 the direction of which is generally parallel to the coast outlines of 



