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Structure of Rocks from the Caucasus. 



[May 5, 



the same may take place also with granitic rocks. Still, even if this 

 mode of metamorphism has occurred, there is some reason to believe 

 that it dates usually, if not invariably, from a very remote period. 

 We cau, however, in my opinion, venture to assert that these Cau- 

 casian rocks, after they had assumed a crystalline condition, under- 

 went great pressures, regional rather than local in their operation, 

 which to some extent crushed the constituents, and gave rise to cer- 

 tain mineral changes. It seems then a legitimate inference that in 

 this part of the Caucasus, as in the Alps, the fundamental rocks con- 

 sisted of crystalline rocks of more than one type, at a period long 

 anterior to the operation of the pressures which folded this part of 

 the earth's crust and upreared the mountain range. 



(3.) The huge mass of Elbruz appears to consist mainly of volcanic 

 rock, and is crowned by two crater-peaks almost equal in height. Of 

 these the eastern, which is believed to be very slightly the lower of 

 the tw r o, was ascended for the first time on July 31st, 1868, by Messrs. 

 Freshfield and Moore. On this occasion the western summit was so 

 entirely concealed by clouds that its existence was not even suspected. 

 The western summit was first ascended on July 29th, 1874, by 

 Messrs. Grove and Walker. Its " crater considerably exceeds in size 

 that on the twin summit, and is probably about f of a mile in 

 diameter. The wall is perfect for some two-thirds of its former 

 circuit, but on the south-west side a vast piece has fallen away, and 

 a great glacier now flows down from the gap." The little peak 

 forming its highest point, juts up on the north-eastern segment of 

 the limb. Its height above the sea, according to the Russian survey, 

 is 18,-526 feet, the eastern summit being 95 feet lower. The col 

 between the two summits, according to Mr. Grove, is about 17,350 

 feet. The specimen collected by Mr. Walker was from the highest 

 rocks traversed on the western peak, perhaps about half-way between 

 the col and the summit. It is a rough slab of a grey lava, with 

 occasional small irregularly-shaped vesicles, and scattered crystals of 

 a whitish felspar up to about \ inch in length. The weathered parts 

 are of a lightish-brown colour. 



Microscopic examination shows that the rock has a clear glassy 

 base crowded with minute lath-like felspar microliths, apparently 

 oligoclase, and occasional specks of opacite and aggregates of ferrite : 

 possibly some minute granules of a pyroxenic mineral are present. 

 To the same epoch of consolidation may belong some occasional 

 elongated crystals of a light-coloured hornblende, but this is uncertain 

 — there are a few grains of iron oxide, probably hematite. The larger 

 crystals in the slide certainly belong to an anterior consolidation — these 

 are (1) a dark brown hornblende, often with rounded outline, and 

 sometimes blackened with included opacite ; (2) a felspar, which 

 generally resembles labradorite or andesine, but in one or two cases 



