320 



Prof. T. G. Bonney. On the 



[May 5, 



occasionally tend to range themselves perpendicularly to the lines of 

 cleavage. They are generally minute, sometimes stained internally, 

 both ovoid and irregular in form, usually containing fluid, and with 

 bubbles which, as a rule, are about one fourth the volume of the 

 cavity, but not rarely exceed this. Some of the large grains exhibit 

 the usual indications of being in a state of strain. 



The mica is brown, greenish, or colourless. The first and second 

 are biotite, more or less altered. The colourless mica resembles 

 muscovite, but I think that at any rate some of it is a magnesia- 

 potash mica, possibly hydrous, a secondary product after biotite, the 

 iron having separated out. This often remains between the cleavage 

 planes in rods and plates. Possibly some of the smaller flakes of 

 mica may be altogether of secondary origin, but 1 have no doubt that 

 most of it, including all the larger flakes, is an orginal constituent. 

 These flakes often afford marked evidence of mechanical disturbance. 

 They are bent, twisted, crumpled, and in some cases crushed up. 

 Portions of them, viewed with the polarising apparatus, have a peculiar 

 " powdered " look, which I find very characteristic of a mica that has 

 been to a certain extent crushed in situ, so that, while the general 

 outline of a crystal is preserved, there are constant ruptures of con- 

 tinuity and slight displacements of the constituent parts. A few 

 small mineral granules also occur in the slide ; some I am disposed to 

 refer to epidote, others to a very impure garnet. 



(2.) The specimens from Guluku were collected, partly in situ at a 

 height roughly of 14,500 or 15,000 feet above the sea, partly from a 

 moraine, as above-mentioned, on the Urban glacier; hence they repre- 

 sent a considerable mass of the mountain below the level just men- 

 tioned. The highest point of Guluku is about 16,500 feet above the 

 sea. 



(A.) From the highest rocks reached. A small fragment of rock 

 with indications of a slight cleavage, consisting of a porcelain-white 

 mineral irregularly mottled with one of a pale pistachio-gTeen colour. 

 The former on microscopic examination proves to be a plagioclastic 

 felspar, considerably decomposed, but in parts showing very clearly a 

 lamellar twinning. The extinction angles are generally rather small, 

 probably oligoclase predominates ; microlithic flakes of a micaceous 

 mineral, and other decomposition products, are frequent. The other 

 mineral is an epidote, varying from a pale yellow tinge to colourless, 

 and rather impure. It occurs in aggregated and sometimes rather fan- 

 like groups of longish crystals. There are also a few spots of a 

 serpen tinous or chloritic mineral. The mechanical disturbance of the 

 rock is obviously posterior to the crystallisation of the felspar, as its 

 crystals are cracked and even sheared, but, at any rate in the main, 

 prior to that of the epidote. 



Guluku (B). — A small fragment of a coarse gneissose rock, evi- 



