597 



rocks as a motive power was, I feel confident, suggested by some one 

 (the name of Mitscherlich or Laplace is somehow connected in my 

 memory with it) many years ago, certainly before 1833. As regards 

 the course of my own ideas, it was simply this. When I first read 

 your book I was struck with your views of the metamorphic rocks, and I 

 began to speculate how and why the mere fact of deep burial might 

 tend to raise the temperature to the required point. Three modes 

 occurred : 1st. development of heat by condensation ; but this cause 

 seemed somewhat feeble, and not very clear in its mode of action, 

 since at every moment an equilibrium of pressure and resistance is 

 established : 2nd. plunging down into an ignited pasty mass ; here, 

 however, considering the excessive slowness of the process, it oc- 

 curred to me that there would be plenty of time for the ignited matter 

 below, not merely to divide its caloric v/ith the newly superposed mass, 

 but to take up fresh from below, and thus to establish a regular gra- 

 dation of temperature from below upwards ; and this led to the 3rd 

 and more general view of the matter, which is that of the variation 

 of the isothermal surfaces, as stated in my former letters." 



It was, however, the perusal of Mr. Lyell's 4th Edition which led 

 to the final development of the theory. 



Sir John Herschel then observes : " When people think inde- 

 pendently at different times, and excited by different original sub- 

 jects of consideration, bearing on one more general object, if their 

 ideas converge towards one vie^/f of the matter, it is a proof, that 

 there is something worthy of further inquiry; and if they think 

 to any purpose, it is hardly possible but that many points will 

 occur to the one which do not occur to the other ; and that so a 

 theory may branch out and acquire a body much sooner than it 

 would do by the speculation of one alone ; and indeed such is, in 

 some degree, the case in the present instance. Babbage, for exam- 

 ple, has speculated not only on the heaping on of matter in some 

 parts, but on its abstraction in others as a cause of variation in the 

 isothermal surface, and justly. It is the case of the algebraic passage 

 from -|- to — passing through 0. In envisaging (as the French 

 call it) the question algebraically, the cases could not be separated. 

 Again, he has confined himself to the pyrometric changes in the solid 

 strata, while I have left these out of view, and relied on what I think 

 to be a far more energetic and widely acting cause — the variation of 

 pressure, and the infinity of supports broken by weight, or softened 

 by heat, to produce tilts. Both causes, however, doubtless act, and 

 both must be considered in further detail. The former alone may 

 account for the phsenomena of the Bay of Naples, the latter must I 

 think be called in to account for those of Scandinavia and Greenland, 

 and of the Andes. 



" I would obsei've that a central heat may or may not exist for our 

 purposes. It seems to be a demonstrated fact that temperature does, 

 in all parts of the earth's surface yet examined, increase in going 

 down towards the centre, in what I almost feel disposed to call a 

 frightfully rapid progression ; and though that rapidity may cease, and 

 the progression even take a contrary direction long before we reach 



