252 Transactions of the Boyal Society of Soutli Africa. 
it, and there are pieces of sandstone and quartzite like some of the Fish 
Eiver beds. These hard rocks are occasionally traversed by veins of very 
fine-grained brown, red, or black jaspery rock which is seen under a high 
power to consist of minutely crystalline quartz crowded with opaque 
ferruginous particles. The veins vary in thickness and may be as much 
as an inch in width ; they can be traced for several yards across individual 
outcrops. 
Some very fine-grained whitish rocks resembling porcellanite form part 
of the eastern wall. They are thin bedded and have a conchoidal fracture. 
In thin section (196 E) one of these rocks is a very fine-grained semi- 
opaque white material with numerous clear areas at most 0-02 mm. across 
and either roughly circular or polygonal in shape, showing the characters 
of chalcedony. A few very small splinters of quartz are enclosed in this 
matrix. The successive layers are due (1) to the greater or less abundance 
of the opaque dusty material, and (2) in some cases to the abundance of 
quartz splinters. A section through a hard red fine-grained rock from the 
southern side of the mountain (203 K) shows an abundant red ferruginous 
base with numerous rounded and angular grains of quartz up to 0*5 mm. 
in diameter, and a smaller number of grains of orthoclase, plagioclase, and 
line-grained sedimentary rocks ; this rock differs from the breccias above 
described only in the uniformly small size of the recognizable fragments 
in it. 
A breccia from the south side of the mountain (199 E) contains pieces 
of microcline in addition to the minerals mentioned above ; the matrix is in 
places formed by quartz grown on to rounded grains of quartz in crystal- 
line continuity with them and enclosing fragments of sedimentary rocks 
and gabbro, etc. 
The obvious character common to all these rocks is that they do not 
contain recognizable fragments of larva. Material of sedimentary origin 
forms by far the greater part of the breccias and tuffs — if the latter term 
can be used in this connection — and the recognizable igneous rocks and 
minerals present were derived from holocrystalline rocks of deep-seated 
origin, not from lavas or ordinary dyke rocks. Though the rocks 
described above are few in number they probably represent the material 
forming the ring-shaped mountain wall very fairly, for a careful search 
was made for different kinds of rock and the several varieties found were 
taken for more detailed examination. Igneous materials in these rocks 
have to be searched for, and the only place where they are at all con- 
spicuous is one of the southern satellite pipes. I found it difficult to 
make sure that recognizable pieces of granite occur in the breccias of the 
mountain ; some small fragments seen might have been granite, but that 
rock is certainly rare in the breccia, though the chief components of 
granite are abundant. Whether part of the matrix is derived frorn an acid 
