Miss C. A. Raisin—Geology of Africa. 437 
lenticular, rather broken-looking, fragments with shaded border, the 
edge of the larger grains is cracked, and thin wavy laminz of mica 
and hornblende sometimes traverse the slide. These characters have 
been often described and are doubtless due to subsequent pressure. 
Other examples shew a clear undisturbed intercrystallisation of the 
constituents, which are chiefly a quartz-felspar mosaic, large felspars 
(microcline or kaolinised plagioclase) and well formed biotite not 
streaked out in layers. The appearance of banding, and some 
elongation of the grains, may perhaps be attributed to a movement 
of the original magma, and the structure thus may be related to 
that which has been described from the districts of the Lizard and 
of Sark. 
The diorites obtained from near Assouan have rather the character 
of intrusive masses, and may be connected with the dykes which 
have been described as cutting the basal series. From the second 
cataract, in addition to the felspar-hornblende rocks, which appear to 
be dykes, some well-marked clear diorites occur, possibly belonging 
to an old crystalline series, and the description suggests that they 
are parts of large masses, forming the typical rocks of the district. 
The hornblende in all appears secondary, affording illustrations of 
various modes of its development from augite. A crushed diabase 
was brought from south of Sarras and also a chlorite-schist.’ 
The specimens from undoubted dykes include basalt and micro- 
granite (Assouan), porphyrite (Kalabsheh) and other varieties. 
One point of interest is the fairly large number of rocks from near 
Assouan which are connected with the mica-trap group [85, 36, 26]. 
They vary from somewhat micaceous diorites to well-developed 
kersantites ; they have often an orientation in the mica, and lead up 
to specimens which are really mica-schists in appearance. It is 
interesting to have further illustrations of this group of not very 
common igneous rocks, which had already been shown by Professor 
Bonney to exist in the district, in his description of specimens 
brought by Sir W. Dawson.? 
A schistose character is also found in certain hornblende rocks 
which are described as dykes [32, 34]. They are certainly igneous, 
and consist of plagioclase felspar and green dichroic hornblende.* 
The absence of pressure effects and the appearance of quiet crystal- 
lization suggests that the foliation again here may be a fluxional 
structure. A dyke from Elephantine [33] is a rather exceptional 
rock, of a dull red colour, the slide consisting of plagioclase felspar, 
of a brown, probably ferriferous hornblende, allied to arfvedsonite, 
1 Q.J.G.S. 1891, vol. xlvii. p. 497, Prof. Bonney and Major-Gen. McMahon on 
the Crystalline Rocks of the Lizard District. Q.J.G.S. 1892, vol. xlviii. pp. 132-138, 
Rey. E. Hill and Prof. Bonney, on the Hornblende Schists of Sark. 
2 This specimen may possibly be related to a chlorite-schist which has been 
described from the borders of Abyssinia. 
3 Grou. Mac. 1886, p. 106. 
4 The hornblende is in clear rounded grains in one slide, which also contains a 
light-coloured augite similar to a variety shown to me in a Hebridean diorite by 
Prof. Bonney. Another of the hornblende-felspar rocks, as Prof. Bonney also pointed 
out, bears some resemblance to a foliated diorite in Guernsey. 
