248 
Transactions of the Boyal Society of South Africa. 
Gubib, only Dr. Schenck's accounts give any details, and other references 
to its structure are made in terms of his descriptions. 
The earliest of Dr. Schenck's descriptions is that Geitsi Gubib is a 
mountain of porphyritic rock ; the next f is similar ; the third | gives the 
results of a microscopic examination of the rocks, w^hich appeared to prove 
them to be porphyry-tuffs and the mountain to be a " porphyrischer 
Stratovulkan." He says that the ground mass apparently consists chiefly 
of siliceous cement with dust-like particles amongst which iron oxide is 
conspicuous, and that this matrix contains fragments of orthoclase, plagio- 
clase, quartz, and magnetite ; in the hard, compact rocks the latter decrease, 
but in the arl^ose-like types they are more abundant ; many of the tuffs are 
said to contain pieces of granite. His description of the mountain itself is 
that it " rises as an isolated cone-shaped mass of rock from the Fish Eiver 
plain, and has a basin-shaped depression in the middle towards which the 
outer wall slopes steeply, and which has a deep valley of erosion going 
south," so that the conclusion is that Geitsi Gubib is a stratified volcano 
with a still well-preserved crater " which retains its shape because the 
surface of the land has not suffered great changes since the volcano's 
origin, and because the tuffs are able to resist denudations strongly on 
account of their siliceous cement. Geitsi Gubib therefore may well be the 
oldest well-preserved stratified volcano, the age of which cannot be fixed 
(we can only say that it is post-Carboniferous), but so far as the nature of 
the rocks goes it is older than the known Tertiary and post- Tertiary 
volcanoes." 
In another place ^ he calls the depression a caldera, and the outlet 
Siidwest Afrika," Berlin, 1898, a collection of 96 photographs and a map, Dr. Kehbok 
gives views of the lower waterhole in the valley leaving the mountain, a good view of the 
" crater " from the north, and one of the beacons on the top of the mountain. 
InL. Schultze's " AusNamaland and Kalahari," Jena, 1907, p. 139, there is a reference 
to the mountain in terms of Schenck's work, and there is a very good view of the mountain 
from the south on Plate VII., and another of the valley draining it, showing the inward 
dip of the Fish River beds and of the tuffs themselves ; on p. 139 there is a view of the 
"crater." Dr. Paul Eange on pp. 126-7 of Monatsb. d. deutsch. geol. Gesellsch., 1909, 
calls it a " Quarzporphyrstock," and remarks that fragments of the Karroo beds are 
apparently enclosed by the tuffs of the old " Stratovulkan " ; and a similar reference by 
him is in Trans. Geol. Soc. S.A., 1910, p. 6. In "Geologic des deutschen Namalandes," 
Berlin, 1912, Dr. Range mentions the mountain again, quoting Schenck's description, and 
also the probable presence of Karroo rocks in the tuffs. 
These appear to be the only references to the geology of Geitsi Gubib besides Dr. 
Schenck's descriptions, though the mountain itself must be mentioned frequently else- 
where. Its position is marked in the map attached to Chapman's "Travels in the Interior 
of South Africa," 1868, though somewhat too far west. The account in Stromer von 
Reichenbach's " Geologic des Deutschen Schutzgebiete in Afrika" is taken from Schenck. 
* " Zeitschr. d, deutsch. geol. Gesellsch." 1886, p. 236. 
t " Verhandl. d. Deutschen Geographentagen," 1893, p. 161. 
I "Zeitschr. d. deutsch. geol. Gesellsch." 1901, p. 54. 
§ " Verhandl. d. XIII. Deutsch. Geographentagen zu Breslau," 1901, p. 157. 
