1887.] 



Structure of Mocks from the Caucasus. 



321 



dently containing a considerable amount of a darkish mica, on the 

 weathered surfaces reddish-brown (the " red band "). Under the 

 microscope it is seen to consist chiefly of biotite, felspar, considerably 

 decomposed, in part at least plagioclase, and quartz. The biotite is in 

 places altered into a greenish chloritic mineral, in others is "bleached " 

 by parting with its iron. A white mica, however, which occurs in 

 good sized flakes, appears to be an occasional original constituent. 

 The rock has evidently been much crushed. The quartz is cracked 

 and displaced, the felspar has been broken up, and parts of the 

 original crystals are now occupied by a sort of irregular mosaic or 

 mixture of felspar, quartz, kaolin, and white mica. The felspar 

 crystals are occasionally interrupted by roundish inclusions of quartz, 

 such as one often sees in the oldest gneisses. These may be of 

 secondary origin, but I find nothing to prove it. The original quartz 

 grains, where adjacent to the crushed felspar, appear to have been 

 augmented by secondary deposits of quartz in optical continuity. 

 The mica in parts of the slide shows marked indications of mechanical 

 disturbance, and a reddish garnet at the edge has been distinctly 

 crushed out, as is more fully described in (C). 



Guluku (C). — This specimen, in shape roughly a right-rhomboidal 

 prism about V X V X j^", has for its larger faces parallel joint 

 surfaces; two others are "sheen surfaces," parallel with which the 

 fractured faces exhibit a.foliated structure. The rock appears to be a 

 strong rather compact mica-schist, dark in colour, with a few very 

 thin lighter- tinted bands. 



The principal minerals are quartz, mica, garnet, iron oxide, and a 

 quantity of a brownish mineral, sometimes very fibrous. The quartz 

 occurs mostly in granules of moderate size, occasionally including a 

 little minute rutile (?) and mica. It is on the whole fairly clear, but 

 here and there cavities are pretty numerous. These frequently con- 

 tain bubbles, which, though very variable in relative size, are gene- 

 rally smaller than in the Tau Tetnuld rock, perhaps commonly about 

 one-sixth or one-seventh of the whole volume. The mica constituent 

 is chiefly biotite or its alteration products, often a greenish chloritic 

 mineral, sometimes a whitish hydrous mica, both with interlamination 

 of iron oxide. The garnet is colourless in thin slices, and occasionally 

 exhibits, along cracks, alteration for a short distance into a chloritic 

 mineral. Some of the larger granules of iron oxide are hematite. 

 The pale-coloured rather filmy or fibrous mineral is certainly in some 

 cases a secondary product after a felspar, the usual aggregate of a 

 minute micaceous or kaolinitic mineral. Other parts, however, con-, 

 si sting of narrow undulating bands of an aggregated fibrous mineral, 

 like a small lock of wavy hair, I was at first disposed to regard as 

 fibrolite, but after repeated examination I am unable to decide. 

 They resemble in some respects a fibrous mica, but their extinction 



