1887.] Structure of Rocks from the Caucasus. 323 



Guluku (F). — Section from one of two specimens representing 

 numerous large blocks of granite fallen from west side of Guluku. 



This is a fairly coarse-grained rock, chiefly consisting of a white 

 felspar and dark mica, not rich in quartz. The mica is mainly 

 biotite in good preservation. There is also a certain amount of a white 

 mica which appears to be an original constituent and to belong to 

 the muscovite group. The felspar is occasionally replaced by kaolinite 

 and micaceous minerals, but much of it is in good preservation ; 

 sometimes one part of a crystal is reduced to an '"earthy " condition, 

 while the rest is quite fresh. Most of the crystals show the twinning 

 of plagioclase, generally on the albite type, but occasionally on the 

 pericline. One of the grains appears to be microcline. I have 

 measured the extinction angles of several parallel lamellae ; it is 

 difficult to get very satisfactory results, but, as in two of the best 

 cases, they appear distinctly too large for albite, between 30° and 40°, 

 the felspar is probably oligoclase. There are two or three small 

 colourless garnets, with probably a little apatite. The quartz contains 

 cavities, in which small bubbles are usually present, about one-sixth or 

 one-seventh of the volume. 



This rock has evidently been subjected to a certain amount of 

 mechanical disturbance since its consolidation. The quartz grains 

 are cracked and show strain-polarisation. The felspar lamellae are 

 occasionally bent, now and then cracked across, but the effects are 

 slight compared with the other cases. 



Mr. Donkin also collected a specimen some miles further down the 

 valley which resembled the rock in a neighbouring cliff. This ap- 

 pears to be a reddish rather fine-grained felspathic granite, but as it 

 was not obtained in situ I have not had a slice prepared. 



The evidence of these specimens does not appear sufficient to 

 warrant any positive statement as to the origin of these Caucasian 

 rocks. The structure of one (D) seems rather to favour the idea of 

 its having been a true granite {i.e., an igneous rock) ; the same is 

 true also of (F) ; while in another (B) there is a structure, which 

 I have some reason to think characteristic of the Archaean gneisses.* 

 But in the present state of our knowledge it w 7 ould be unsafe 

 to rely too much upon the latter criterion, because we do not 

 yet know what modifications may be introduced by subsequent re- 

 arrangement of mineral constituents. It has indeed been proved in 

 the case of hornblendic rocks, that the original structures, character- 

 istic of crystallisation from a state of fusion, may be wholly obliterated; 



* The same difficulty exists in the case of some of the more highly crystalline 

 rocks of the Alps. Favre (' Eecherches — Chaine du Caucase,' p. 70) states that the 

 central part of the Caucasus is " granite," which he compares with the protogine of 

 the Alps, with a considerable belt of crystalline schists on the north, and an inter- 

 mittent one of the same, followed by slates, on the south. 



