Correspondence — Prof. T. G. Bonne//. 383 



Prof. Bonney described specimens brought by Col. Feihlen from 

 Norway, the Kola Peninsula, and Novaya Zemlya. From an 

 examination of the rocks obtained in situ in the last-mentioned 

 region, Prof. Bonney confirms Col. Feilden's suggestion that the 

 Kolguev erratics may have come from Novaya Zemlya. 



2. " Extrusive and Intrusive Igneous Kocks as Products of 

 Magmatic Differentiation." By Prof. J. P. Iddings, For. Corr. G.S. 



The author, after pointing out the propositions concerning differen- 

 tiation of magmas upon which he is in agreement with Prof. Brogger, 

 discusses the points of difference, and describes the relation of the 

 igneous rocks at Electric Peak to all of those which took part in the 

 great series of eruptions which occupied almost the whole Tertiary 

 period, and spread themselves over a vast territory in Montana, 

 Wyoming, and Idaho. In Tertiary times the eruptions were at first 

 largely explosive, and the accumulation of tuff-breccia formed a chain 

 of lofty volcanoes, comparable with the Andes in size as well as in 

 the nature of their material (andesite and andesitic basalt). After 

 considerable erosion of these volcanoes, gigantic fissure eruptions 

 flooded the region west of the denuded volcanoes. The massive 

 lava-streams which welled from these fissures consisted at first of 

 rhyolite with an average silica-percentage of about 74, alternating 

 occasionally with basalt ; but the great bulk of the basalt was 

 poured out immediately after the rhyolite from fissures still farther 

 to the west and south-west. In the case of these extrusive rocks, 

 whose volumes are of such magnitude, the evidence drawn from 

 the succession of their eruptions and from their composition is of 

 a higher order than that derived from the smaller and more localized 

 eruptions ; and it is upon evidence of this order that the author 

 ventured to enunciate the principle that in a region of eruptive 

 activity the succession of eruptions in general commences with 

 magmas representing a mean composition and ends with those of 

 extreme composition. 



COEBESPOUDElsrCE. 



MR. DTJGALD BELL AND THE PARALLEL ROADS OF GLENROY. 



Sir, — Did time allow, it would be easy to show that Mr. Dugald 

 Bell's criticism of my remarks on the Parallel Koads of Glenroy 

 (page 319) is a typical instance of forensic advocacy. But I am 

 leaving England for the Alps, so that Lochaber must rest in peace. 

 Whether on my return it will be worth while spending time in 

 travelling over well-worn paths, is a doubtful matter. Nothing can 

 be settled about Glenroy till some fresh evidence turns up. Mean- 

 while Mr. Dugald Bell may rest assured that I am not frightened at 

 being in a minority. I have lived and been a geologist long enough 

 to have watched the apparition and the fading away of many 

 Brocken-spectres, not in our science only. ry q^ Bonney. 



July 3rd, 1896. 



