Miss C. A. Raisin— Geology of Africa. 439 
mass was kept for a long time at a low temperature, the peculiar 
character of the diorite may have resulted from the “stewing” 
undergone, and so may possibly be a kind of contact metamorphism. 
One specimen of an andesite—a pale peach-coloured rock— 
contains fairly large fragments, among which clear glassy quartz is 
conspicuous. The microscope shows that these included parts are 
a foreign element (quartz and felspar) evidently belonging to a 
rather coarse granite or granitoid rock. ‘They vary in size, from 
4 inch to small microscopic pieces. The edges, sometimes straight 
along cleavage boundaries of the felspar, seem sharp, and show no 
evidence of melting down, unless a narrow border with lighter 
streaks indicates such an effect. The ground-mass is evidently 
glassy, exhibiting fluidal structure, and it seems to represent an 
andesitic magma, which has broken through and enclosed fragments 
of a granite or gneiss. 
Specimens of rocks collected from near the base of the overlying 
sandstones at Abu Seir and at Kormoon are very rotten, but appear 
to be much decomposed basalts or dolerites, now consisting mainly 
of a ferruginous mass with a pinite-like mineral. They might 
represent old lavas which flowed above the massive igneous series ; 
but they more probably are dykes intrusive in the underlying rocks 
or more or less along the junction plane. 
Among the lower crystalline rocks the signs of deformation are 
not very marked, although on some specimens the great earth- 
movements or local disturbances have left their impress. Thus a 
much crushed and consolidated diabase occurs at more than one 
place ;’ some gneisses of Assouan show structures due probably to 
_ pressure of ancient date, and some other illustrations can be found ; 
but on the whole the disturbance is certainly not extreme. 
The overlying sedimentary rocks appear to be almost horizontal 
and undisturbed. ‘They are in nearly all cases sandstones of the 
Nubian type and consist mainly of quartz and felspar (often 
microcline) frequently forming a true arkose. Thus they were 
evidently derived from the disintegration of the lower crystalline 
rocks. 
Sands cemented by iron oxide form a kind of ‘pan’ such as has 
been noticed in previous explorations.? This character often appears 
to be associated with a hot, dry climate and a short but heavy rainfall 
on a sandy country. The accretion of secondary quartz pyramids 
is also illustrated,’ and the grains are rounded and angular, including 
probably a mixture of materials transported by wind and by water. 
The drifting of desert sand would doubtless smooth rock-surfaces, 
and some beautifully polished specimens have been brought from 
the Nile Valley. The glaze* may indeed have been added to by 
1 One specimen is from far south beyond Sarras; others from further north near 
Wady Halta. 
% Mineralog. and Petrogr. Mittheil. G. Tschermak. 1886, vol. viii. p. 24; Across 
East African Glaciers, Dr. H. Meyer, p. 347. 
3 See J. A Phillips, Q.J.G.S. 1881, vol. xxxvii. pl. ii. figs. 1, 2, 3, 5. 
* See M. L. Lartet, Géol. de Palestine, p. 57 ; similar specimens are described by 
J. C, Hawkshaw, Q.J.G.S., vol. xxiii. p. 116. 
