Correspondence — Rev. O. Fisher. 45 



ON THE ELEVATION OF MOUNTAIN CHAINS. 



Sir, — In reply to the slight notice with which Mr. Scrope has 

 honoured my speculation on volcanic action/ I can assure him that 

 nothing was further from my intention than to claim as original 

 what I had learnt from him. It was merely for the sake of brevity 

 that I omitted a reference, which I thought every one could supply. 

 When my paper was read, I used the words, " With respect to the 

 raising of ejectamenta in a fissure, it is clearly proved by Scrope, in 

 his work on volcanos, that the force to which it is due is the expan- 

 sion of aqueous vapour when relieved from pressure." I regret that 

 I did not transfer the sentence in full to your pages. 



It will, however, be perceived that although I am indebted to Mr. 

 Scrope for my ideas of the nature of a volcanic eruption, my specula- 

 tion as to its cause differs from his theory. 



He attributes the elevation of mountains and the trains of vol- 

 canoes which often accompany them, to local changes of temperature. 

 "The results of such a local change of temperature would seem to 

 be, first, the dilatation — whether or not amounting to fusion — and, 

 con sequent, upward pressure and bodily rise of the expanding matter 

 beneath the centre or medial line of the area affected, but without 

 producing its outward extravasation there; and, secondly, and at 

 the same time, the upward rush and (sooner or later, probably) the 

 external eruption of portions of this heated and fluidified matter 

 through fissures formed towards the margin of the elevated area, and 

 ranging in parallel lines on one or both sides of its central axis of 

 maximum upthrust." ^ It appears, then, that the motive power, in 

 Mr. Scrope's opinion, is the pressure from below of matter expanded 

 by an accession of heat. 



I, on the other hand, conceive the elevation of the mountains to be 

 owing to the contraction of the general mass of the earth within its 

 already cooled crust, and suspect a diminution of pressure beneath 

 moimtain ranges on account of their being partly supported by their 

 lateral abutments. I conceive the diminution of pressure so caused 

 to induce liquefaction of the subjacent plutonic mass ; so that erup- 

 tion takes place through vents prepared for it — not by the upward 

 pressure of increasingly heated matter, as supposed by Mr. Scrope ; 

 but by the crumpling of the crust through lateral pressure caused by 

 a general cooling of the globe. To my mind the difference between 

 these views amounts almost to an interchange of cause and effect. 

 Harlton, near Cambridge. 0. FlSHEK. 



FISHER.— DENUDATIONS OF NORFOLK. 



Sib, — Under this heading your number for December contains a 

 paper by the Eev. 0. Fisher. The opening sentence is — " Upon the 

 land-surface a certain amount of the fine material is beino- carried 

 into the rivers, and by them deposited at the heads of the Broads, or 

 where such do not exist, in the sea. This denudation hy pluvial action 

 is undoubtedly greater where the land is under the jolough than it 

 would be otherwise." The wildest subaei'ialist will require nothino- 



1 Geol. Mag. Vol. V., p. 493. 2 Scrope's Volcanos, lS62,p. 273. 



