Theories of the Earth^s Physical Evolution. 323 



present distribution of subterranean temperature ? He took the 

 highest reasonable value of initial temperature, viz. 7000"^ Fah., 

 which may be regarded as extravagantly high, in order to avoid 

 any objections which might be made by geologists. And he took 

 the mean present rate of increase of temperature in descending, 

 around which nearly all results of observation tend to cluster, viz. 

 one degree for fifty-one feet; while the conductive power of the 

 commoner kinds of rock and their specific heat were determined by 

 experiment. From these data he deduced the period of cooling, and 

 the present distribution of temperature within the globe, arriving at 

 the following results. The period since the globe solidified [suppos- 

 ing it to have become solid throughout simultaneously"^'] cannot be 

 less than twenty million, nor greater than four hundred million of 

 years. Below the depth of one hundred and forty miles the amount 

 of cooling has been immaterially small — the nucleus being nearly as 

 hot as ever. Various criticisms have been made respecting the 

 quantities used by Sir William in this computation, but after making 

 every allowance for possible errors, the following conclusion still 

 remains from his application of the theorem : That no cooling has 

 taken place to any important extent to a greater depth than three 

 hundred miles. The ratio of the cooled crust to the uncooled 

 nucleus thus deduced would be about the same as that of the peel of 

 a fair-sized Havana orange to the edible interior. 



If this argument of the most eminent physicist now living 

 cannot be invalidated, what becomes of Mr. Mallet's hypothesis ? 

 It fails utterly by the destruction of his most important premiss. 

 He assumes a large amount of cooling and a large amount of 

 consequent contraction. It results from Sir William's argument 

 that there has been only an insignificant amount of either. 

 The difference is one which admits of no compromise ; for the 

 two quantities do not lie within the same order of magnitude, 

 and by no possible revision can they be made conformable to 

 each other. One of the two must be utterly wrong and untenable. 

 The exact point of conflict will be found in the different inter- 

 pretations which they place upon the observed rate, or rates, of 

 increase of temperature with depth. Sir William adopts without 

 question the view that it is an index of the aboriginal heat 

 still remaining within the earth at the present stage of its secular 

 cooling. Mr. Mallet holds that it is an indication of heat developed 

 by secondary causes, produced by a stage of cooling incomparably 

 more advanced. It may be noted here that Mr. Mallet's view is a 

 " gratuitous assumption " ; the very kind of assumption which he 

 asserts to be unnecessary to his hypothesis. The older view, 

 although it may not be capable of the rigorous demonstration whicli 

 physical problems sometimes require, rests upon some substantial 

 evidence. We are indebted to no one more than to Mr. Mallet for 

 probable evidence that the seat of volcanic action is not very deep ; 

 and if the heat encountered in mines, and artesian wells, is not 

 good evidence of high temperature near the surface, volcanos are. 



* The sentence enclosed in square brackets is inserted by the Eeviewer. — Edit. 

 Geol. Mag. 



