438 Miss C. A. Raisin—Geology of Africa. 
of pale violet-coloured augite and dense interfelted granular masses, 
including an acicular mineral (?a zeolite) and analcime. If these 
patches replace original crystals, their outlines are not now dis- 
tinguishable, but the aggregates very possibly resulted rather from 
alteration of a ground-mass filling up interspaces. The rock bears 
some resemblance to the augite syenite described by Brogger from > 
Norway; although the presence of nepheline or elzolite cannot be 
proved,’ and, as stated just previously, the mass very probably was 
not even holocrystalline, but possessed an andesitic matrix. ‘The 
augite has definite outlines, and probably was formed early, and 
hornblende was developed partly by conversion of the augite, partly 
as a zone surrounding it. 
Certain syenitic or granitic rocks from near the second cataract 
appear from the description to be intrusive in the diorite, and the 
evidence from some junction specimens, although not decisive in 
single examples, is in favour of this view. [113.] In one, the granite 
is coarsely crystalline, but the diorite forms a small piece, which 
probably has been included in the heart of the intruding mass. In 
the fragment, greenish hornblende is in the form of aggregates of 
small crystals confusedly orientated, enclosing in a few cases a 
partially decomposed core of augite. But in addition to characters 
which are common in a hornblende-diabase or uralite, a further 
alteration can be traced. Small crystals of clear brown mica form a 
zone along the border of the original pyroxene. These range roughly 
parallel to the boundary, or they project outwards penetrating the 
adjacent felspar; sometimes a small group occurs surrounded by 
felspar (Fig.1). Thus, as Prof. Bonney has suggested in connection 
with a specimen from Sark, the mica seems to have crystallized at 
the expense of the felspar and the pyroxene.? This secondary 
development possibly may be due to the influence of the intruding 
granite, for it is the kind of change which we generally associate 
with contact metamorphism, and we note here that the mica films 
are smaller than the size of the felspar and pyroxene would lead us 
to expect (see Wig. 1, p. 442). 
[87.| The diorite in another contact specimen also shows peculiar 
alterations. Dull green or brown dichroic crystals, partly of horn- 
blende, partly of brown mica, are scattered with a rough orientation 
within a mass of plagioclase felspar. The felspar is generally 
idiomorphic and has a sharp demarcation between a kaolinised 
interior and a clear external zone (Fig. 2). The intrusive rock is 
a reddish felstone; it consists mainly of a micrographic mass sur- 
rounding and often growing from rectangular cores of dusty felspar, 
which themselves are sometimes zoned. If we may infer, as has 
been pointed out by Professor Bonney in the “‘syenite”’ of Charn- 
wood,’ that for the formation of this micrographic structure the rock 
1 The interfelted mass bears some resemblance to that in a nepheline syenite from 
Montreal, shown to me by Prof. Bonney. 
2 Q. J. G. 8S. 1892, vol. xlviii. p. 131, fig. 3. 
3 Q. J. G. S. 1891, vol. xlvii. p. 105. Ona Contact-structure in the Syenite of 
Bradgate Park, by Prof. T..G. Bonney. 
