104 Prof. T. G. Bonne!/ — Structure of the Bocks of Assouan. 



usual green mineral, a little apatite, and a small yellowish mineral 

 probably spbene. Tbe rather irregular outline of the felspars and 

 the granular aggregation of the quartz resembles in some respects 

 the structure of a gneiss rather than a granite, but it is very probable 

 that this structure is the result of some pressure and crushing after 

 the consolidation of the rock, of which the hand specimen gives 

 indication : in all other respects one would not hesitate to call it 

 a granite. 



(4.) A rather fine-grained noncrystalline rock, speckled pinkish 

 and dark coloured. 



Microsc. — Quartz, felspar (orthoclase, microcline, and plagioclase), 

 with a considerable amount of brown mica, a little apatite and 

 epidote or sphene, with a very little iron peroxide. It is extremely 

 difficult to say whether this be a gneiss or a granite crystallizing 

 after the vein granite type (granulite of Fouque and Levy), modified 

 slightly by subsequent pressure. After all, perhaps, some of the very 

 coarse ancient gneisses may be only crushed and recemented granites. 



(5.) A rather fine-grained holocrj'stalline rock, speckled pinkish 

 and dull green ; general tint inclining to dark. 



Microsc. — The remarks on (4) will apply here, except that there 

 is rather more biotite and apatite. The structure also is more 

 gneissic, but it is very doubtful from indications given by the slide 

 whether we can trust this, and whether the rock may not originally 

 have been a granite. 



(7.) A dark crystalline hornblendic rock, with a slightly fissile 

 structure. 



Microsc. — Holocrystalline ; hornblende and felspar (chiefly plagio- 

 clase), a little epidote (probably) and magnetite, a flake or two of 

 biotite, and a very little apatite. Very difficult to decide upon the 

 origin of this rock. I incline to think it igneous, and a diorite. 



(8.) A rather dark grey and pink, somewhat finely crystalline rock, 

 with fairly distinct foliation. 



Microsc. — Quartz, felspar, — orthoclase, a little microcline, plagio- 

 clase (? albite), — biotite, with a little iron peroxide, and a very little 

 apatite. The structure much resembles that of one of the old 

 Laurentian gneisses, which is also in accordance with the macro- 

 scopic aspect of the rock. 



(11.) A gneiss distinctly streaked with pinkish felspathic and dark 

 hornblendic or micaceous bands ; the former, with occasional rounded 

 felspars, have an aspect suggestive of much crushing. This 

 appearance is fully confirmed by microscopic examination. The rock 

 has evidently been once either a hornblendic granite or a granitoid 

 gneiss, and its present structure is due to great crushing, so that in 

 parts it is like an ordinary fragmental arkose. The dark mineral 

 appears to be chiefly hornblende, though much biotite is also present ; 

 its crystalline grains are mostly small. Orthoclase, microcline and 

 plagioclase were probably among the felspars of the original rock. 

 There is some apatite, also sphene (?), ilmenite (?), and a crystal 

 of a dark brown mineral unknown to me. 



(12.) A very dark distinctly foliated schist, composed chiefly of 



