Geitsi Giibib, an Old Volcano. 
253 
volcanic rock is at present uncertain, it would be difficult from the 
specimens examined to prove either that it had such a source or riot ; one 
can only record the fact that no material seen hitherto in these breccias 
and tuffs resembles ordinary volcanic rocks or those unusual kinds met 
with in the Kimberlite pipes. 
Porphyry- tuff" is a misleading name for the rock ; the fragments of 
felspar are not whole crystals, and they are inconspicuous components of 
the rock; augite is not noticeable in the hand-specimens. Neither in the 
field nor in the laboratory were any lapilli seen. 
In the collection at the S.A. Museum there are two specimens 
of tuff labelled " Eochlitz " and " Eochlitzer Berg " respectively. 
They bear a superficial resemblance to the lighter-coloured red tuffs of 
Geitsi Gubib ; the former are looser in texture, are crowded with quartz 
and orthoclase, and contain obvious fragments of quartz-porphyry. A thin 
section of a Eochlitz tuff is in tiie collection of the Geological Department 
of the S.A. College, and Professor Young kindly let me examine it ; it has 
a tine-grained matrix with crystals and fragments of quartz which are often 
corroded after the manner of the quartz crystals in porphyry. These 
rocks clearly show no other point of resemblance to those of Geitsi Gubib 
than in being clastic rocks. Pieces of sediments and holocrystalline rocks, 
which together make up all the recognizable fragments at Geitsi Gubib, 
are absent from the Eochlitz specimens. 
It is not easy to define the source of most of the fragments in the 
breccia. The most abundant types of rock are very fine-grained sediments, 
some of which, as stated by Dr. Eange, may have come from the Karroo 
beds which possibly surrounded the pipe at a higher level than the present 
surface, but at present it is impossible to be certain of the identification. 
Fragments of quartzite and some shales are like some beds of the Nama 
formation pierced by the neck. 
Joint planes and cracks in the breccias and tuffs are occasionally found 
coated with hyalite, water-clear opaline silica ; small quartz veins are 
frequent, and a few thin veins of chalcopyrite and chrysocolla were seen 
in the tuffs exposed in the valley draining the central basin ; barytes 
occurs rather frequently in small veins and cavities in all the varieties of 
tuff's and breccias. 
In general appearance the Geitsi Gubib breccias and tuffs resemble 
the materials filling pipes and fissures at Saltpetre Kop," Kobe Eiver,t 
and Grenaat Kop,:[: in the Cape Province, though at all these latter 
localities the breccia is coarser than the Geitsi Gubib rocks, and several of 
* Kogers and du Toit, Trans. Phil. Soc. S.A., vol. xv., 11)04, pp. 01-83; and Ann, 
Rep. Geol. Com. for 1904, pp. 41-43. 
t Ann. Eep. Geol. Com. for 1904, pp. 41-43. 
I A. L. du Toit, Ann. Hep. Geol. Com. for 1908, p. 114. 
