442 Miss C. A. Raisin—Geology of Africa. 
Ficure 1. 
Microscopic section from a small (two-inch) fragment of diorite which apparently 
was included within a fairly coarse granite. From Wady Halfa [113]. 
In the diorite, small crystals of biotite, which occur in connection with the horn- 
blende, and imbedded in the felspar, appear to be secondary, and very possibly 
originated as results of the contact alteration. (Compare Q.J.G.S. 1892, 
vol. xlviii. p. 181, fig. 3.) See p. 438. 
a. Biotite. 6. Hornblende probably derived from an original pyroxene. 
The remainder of the slide is occupied by felspar, the part marked by broken 
lines showing plagioclase twinning. (Magnified about 35 diameters.) 
FicureE 2. 
Felstone intrusive in diorite. From near second cataract of the Nile (87]. The 
mciroscopic slide shews the junction of the two rocks, omitting towards the upper 
