Geitsi Giibib, an Old Volcano. 
261 
the contact can be seen in section over 200 feet in height, no doubt the 
verticahty of the junction is maintained all round the roughly circular 
area of fragmental rocks. The latter evidently occupy a "pipe" about 
2 miles in diameter resembling a volcanic neck, but nowhere do they pass 
over this circular boundary. The Fish Eiver beds near the boundary dip 
inw^ards towards it at a low angle, but at a distance of about 300 yards 
their inclination changes in direction and is outwards, so that the contact 
is surrounded by an anticline parallel to it in the Fish Eiver beds and 
about 300 yards distant from it. Whether this structure is continued 
round the north-western side of the mountain is uncertain, but it exists in 
the west, south, and east. The outward dip decreases and disappears 
about a mile south of the southern contact. 
Petrogkaphy of the Kocks. 
Fragments over an inch in length are rarely seen in the rocks of 
Geitsi Gubib itself, though much larger pieces go to form the satellite 
kopjes to the south. The coarser breccias of the mountain are generally 
brown or red, the finer-grained rocks red, yellowish, or white. The highest 
part of the mountam is made of brown breccia with fragments of hard 
shale and red and black jaspery rocks up to nearly an inch in length, 
cleavage faces of felspar are occasionally visible and there are also quartz 
grains, and calcite in small irregular cavities. In thin section (193 E, 
191 E)" this rock is seen to be made of pieces of very fine-grained sedi- 
mentaryirocks and a holocrystalline rock related to the quartz-gabbros, 
together with rounded and angular grains of quartz and of the felspar and 
augite from the gabbros. The plagioclase is fairly fresh, but the augite is 
usually stained yellowish red and is often considerably altered ; in a few 
of the gabbro fragments wedges of micropegmatite are seen between the 
other constituents. The cementing material is mostly quartz and dusty 
stuff, but there are small cavities in the matrix lined with quartz crystals 
and filled in with calcite. The quartz-gabbro fragments differ from the 
rocks of similar mineral constitution intrusive in the Karroo beds in 
not being ophitic ; they are more like some of the plagioclase-augite rocks 
of pre -Karroo age in the north of the Cape Province. 
The red breccias of the eastern and south-western parts of the mountain 
differ from the brown rocks just described in having more abundant red 
ferruginous matter in the matrix (197 E and 198 E). In thin section some 
of the fragments are seen to be cherts with opaque dusty particles in 
small patches surrounded l^y a nearly clear matrix. Orthoclase frag- 
ments are present as well as pieces of gabbro and minerals derived from 
* Figures in brackets refer to slides in the Cape Town office of the Geological Survey. 
