208 T. Mellard Reade — A Cooling and Shrinking Globe, 



the uncovered areas, and especially tlie areas subjected to denudation, 

 for denudation will act, of course, in an opposite manner and depress 

 the isogeotherms, but not at the same rate, as the mean rate of 

 denudation — being over a larger area — is less, as I have shown, than 

 the mean rate of deposition, which is over a smaller area. 



And here I would point out that whether the covering material 

 be aqueous sediment or volcanic ashes or lava sheets, the principle 

 of conservation of the internal heat of the globe is the same. I 

 think, however, that the non-conductive property of ashes would 

 be much greater than ordinary sediments, and combined with lava 

 sheets — whether extrusive and laid down on the surface, or intrusive 

 like the Great Whin Sill — they would bring actual additions of 

 heat as well as matter to the conservative covering. 



As the larger part of the earth's surface, so far as we are enabled 

 to examine it, is on the continental areas covered with aqueously 

 derived sediment, sometimes interstratified with volcanic and 

 plutonic rocks, and the remainder is covered with or consists of 

 igneous rocks, such as granite or basalt, or of old gneiss or meta- 

 morphic rocks made out of any of these rocks, we at once see that 

 we must not limit our view of the earth to that of an inert body 

 cooling from its outer envelope into space, but of such a body 

 subject to secular cooling and to the reactions brought about by 

 meteorological influences and to the irregularities of cooling and 

 consequent expansion and contraction which have taken place from 

 the beginning of geological history, and is evidenced by all the 

 surface phenomena with which we are acquainted. 



The Shell of Compression. 



We will now go back to the point from which we started and, 

 looking at the globe as an inert mass cooling symmetrically into 

 space, try and quantitatively examine whether the shell of com- 

 pression, forced by its own gravitation to adapt itself to the shrink- 

 ing globe, is sufficient to account for the corrugation of the earth's 

 surface, together with its volcanic phenomena, as was hitherto held by 

 most geologists, and still is by some, though in a hesitating sort of 

 way.^ I pi'efer to treat this in a very simple manner, so as to bring 

 it within the comprehension of the ordinary mechanical mind. We 

 have seen that, according to Lord Kelvin's results — in which, I 

 believe, most physicists concur — the practical cooling of the earth 

 only reaches down now about 150 miles. Let us assume that this 

 shell, 150 miles deep, has cooled an average of 1000° since the 

 beginning of Cambrian times. It has, of course, on the hypo- 

 thesis cooled much more at the zone of greatest contraction, 

 but almost nothing at 150 miles deep. It seems to me altogether 

 too ample an allowance for loss of heat by secular cooling in 

 the selected time, but, whether it be too little or too much, 

 it will give us the sense of proportion we require to form our 



1 See Le Conte — " Theories of the Origin of Mountain Ranges," Nature, Oct. 5th, 

 1893, and my examination thereof, entitled "Genesis of Mountain Ranges," Natural 

 Science, Nov. 1893. 



