106 Prof. T. O. Bonnet/ — Structure of the Bocks of Assouan. 



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

 a good deal of biotite and hornblende, rather rich in apatite, some 

 magnetite, some sphene (one crystal rather large), also several long 

 colourless needles, ? sillimanite ; a hornblendic granite, rather poor 

 in quartz. 



(G.) Upper Series at Biggeh (Assouan). 



(9.) A dark rather compact rock, with but slight indication of a 

 schistose structure. 



Microsc. — Holocrystalline ; quartz, felspar (apparently both ortho- 

 clase and plagioclase, but not very well characterized), biotite and 

 hornblende, with a little magnetite, apatite, and sphene. It resembles 

 a hornblende schist, but slightly foliated, rather than an igneous rock. 



(10.) A dark rather compact massive rock with rectangular 

 jointing. 



Microsc. — Holocrystalline, but not coarse, composed of biotite, 

 rather dark-coloured, but partly altered into a greenish mineral, and 

 hornblende with some quartz, and a fair amount of felspar, which 

 seems commonly to be plagioclase ; a little apatite, magnetite or 

 hematite, and sphene. The mica (with perhaps the hornblende) 

 appears to have been the first mineral to crystallize. The rock 

 appears to me to belong to the mica-trap group, and to be a quartz- 

 il'erous kersantite. 



(16.) A dark rather fissile rock, looking more like a bad slate 

 than a true schist. 



Microsc. — At first glance it seems very fragmental, but on careful 

 examination I feel convinced that we are dealing with a crushed 

 crystalline rock, which has consisted chiefly of felspar and horn- 

 blende. There is a fair amount of apatite, and some of the hexagonal 

 crystals appear to have escaped the crushing, or have been sub- 

 sequently formed. I notice a few flakes of brown mica and there 

 are indications of iron oxides. Thus it is now a schistose rock, 

 not of a highly metamorphic aspect, but has been made out of a 

 diorite or a hornblende schist. 



The " coarse dark-coloured porphyrinic rock " from this series 

 (No. 1 of Sir J. W. Dawson's series at page 440 of the volume of 

 this Magazine for 1884), if I have rightly identified the specimen, 

 is holocrystalline and it appears to me to be a true granite. I may 

 add in regard to this " upper series " that, if I am right in my 

 interpretation of (16), the specimens do not suggest to me the 

 necessary existence of a wide gap between them and those of the 

 lower series ; that some appear to me igneous rocks, and that the 

 others, if not igneous, belong to a highly crystalline group of rocks. 



Eemarks. 



I have had to speak with some hesitation as to the nature of certain 

 of the above described rocks. This is always needful in dealing 

 with any series of very old rocks, especially when one has not had 

 the opportunity of examining them in the field, because we are not 

 yet sure of the significance of certain structures and their relation to 



