O. Poulett Scrope — On Lavas. 105 



that attested by a fall in the barometer, as is seen in tho case of 

 Stroniboli. And it is difficidt to believe that this process is re- 

 versed in the interior of tho volcano. On this point I will say no 

 more, but only refer to my paper in this Magazine, Vol. V., p. 537. 



Before concluding, I ought, perhaps, to notice tho challengo 

 addressed to mo by Mr. Fisher (p. 45 supra), who, attributing to 

 mo, with justice, the opinion that " the motive power in the eleva- 

 tion of mountain chains is tho pressure from below of matter 

 expanded by an accession of heat," opposes to this view what he 

 calls the contrary one, viz., that the effect in question is produced by 

 " the crumpling of the crust through lateral pressure caused by a 

 general cooling of the globe ; the elevation of portions of the crust, 

 by this process, occasioning a diminution of pressure and consequent 

 liqueliiction in the subjacent mass." My reply is, firstly, that my 

 view is built solely on recognized facts, such as the proved upward 

 transmission of heat from below the earth's surface, and the unde- 

 niable inference from known laws of physics, that the rate of this 

 transmission, in other words, the subterranean iso-thermal plane, 

 must vary locally with those variations in the thickness and con- 

 ducting powers of the sedimentary deposits on the surface, which 

 we know to be always taking place, (p. 308, Volcanos). I have 

 purposely abstained from theorizing upon any mere assumption, 

 such as that from which Mr. Fisher deduces his view, viz., the 

 secular cooling of the entire mass of the globe, and its " consequent 

 contraction within a solidified crust" — an assumption which, in the 

 present state of our knowledge, is not, I think, sufficiently justified 

 to aflbrd a reliable postulate in geological speculations, though I am 

 quite aware of its popularity and plausibility. At the same time I 

 may observe that this view of the primary cause of the elevation 

 of mountains (which is that of Cordier, Elie de Beaumont, and 

 many others) is not inconsistent with mine, viz., that the direc- 

 tion taken by the lines of fracture in the earth's crust, under 

 the supposed circumstances, and consequently the positions of the 

 alternate areas of elevation and depression, must be determined by 

 the local conditions of comparative tension — that is, of temperature — 

 in the matter beneath, and of the resistance in the crust above ; the 

 elevated areas consequently coinciding with those in which the 

 expansive influence of subterranean heat predominates over the sum 

 of resistances offered by the crust above ; and the depressed areas 

 to those in which these conditions are reversed. That intense heat 

 was the chief agent in the up thrust of the crystalline axes of most 

 mountain chains is proved, I think, by the signs they exhibit of 

 having melted their way into the overlying rocks, which they at the 

 same time appear to have upheaved. While the volcanic or trap 

 rocks, on the contrary, seem to have risen through fissures with 

 scarcely any disturbance or metamorphism of the sides of these. 

 The former I consider the direct effect of subterranean expansion 

 from increase of heat; the latter its secondary effect, due to the 

 opening of lateral avenues of up-flow by the action of the first force. 

 (See Volcanos, ed. 1862, p. 273.) 



