Prof. T. G. Bonney— Structure of the Rocks of Assouan. 105 



mica and a decomposed (?) felspathic mineral ; some of the mica has 

 a brassy lustre. The attempt to obtain a slice of this rock has been 

 a failure, as it is so friable. 



(13.) A hard quartzose schist, with not much mica, and little 

 signs of foliation. 



Microsc. — The principal constituents are quartz, felspars of more 

 than one species, and brown mica. The rock appears to have been 

 squeezed, but not crushed. It has the aspect of a true gneiss, one of 

 the fine-grained kinds that rather resemble quartzites. 



(14.) Rather like (12), but with a felspathic vein. 



Microsc. — Dominant minerals, plagioclase, biotite, hornblende, 

 some quartz, but variable in distribution. Parts of the slide might 

 thus be called hornblendic gneiss, the rest rather hornblendic mica- 

 schist. The rock certainly must now be called one of the above 

 names, but there are peculiarities in its structure whioh prevent me 

 from making any confident statement as to its origin. 



(15.) A rather 'slaty' fine-grained gneiss, related to (13), but 

 more fissile. 



Microsc. — It has a general similarity to (13), but contains more 

 mica. Also it has yielded more to pressure. 



I may add that among the specimens from this series are several 

 rather fine-grained dark mica-schists or very micaceous gneisses, 

 varying between (12) and (15), which are so obviously representatives 

 of the " metamorphic " group of rocks, that I have thought it needless 

 to examine them under the microscope. They have a general re- 

 semblance to schists not unfrequent in the upper parts of the Hebridean 

 series of Scotland, and may be compared with some of those which 

 occur (for example) in the neighbourhood of Gairloch (Ross). 



(B.) Rocks Intrusive (Dykes, etc.) in Series A. 



(1.) A noncrystalline rock, mottled dark green (almost black) and 

 light yellowish, with porphyritic felspar crystals of latter colour, 

 sometimes about one inch long. 



Microsc. — Consists of quartz, felspar — orthoclase, microcline, plagio- 

 clase (albite?) — hornblende, biotite; with a little magnetite, a good 

 deal of apatite in well-defined hexagonal prisms, and a little of a 

 granular yellowish mineral, probably sphene. Thus the rock is 

 a hornblendic-granite ; it is not very rich in quartz. 



(2.) Coarsely crystalline rock, with large pinkish-red felspar 

 crystals, quartz, some whitish felspar and black mica. One of the 

 granites commonly used in Egyptian monuments, and, as is often the 

 case with these, it has a gneissoid aspect. 



Microsc. — Holocrystalline ; quartz felspar, chiefly microcline, with 

 some plagioclase, brown mica, with a little hornblende, a little mag- 

 netite, some apatite, and a yellowish mineral ; sphene, or perhaps 

 epidote. The irregular outline of the felspar and the aggregated 

 granules of quartz resemble a gneiss, but this may be due to subse- 

 quent crushing. 



(6.) A holocrystalline rock, moderately coarse, speckled with light 

 greyish colour and black (mica). 



