Geitsi Giibih, an Old Volcano. 
255 
pebbles or boulders in any of these rocks such as are found occasionally 
in the satelhte pipes of Saltpetre Kop. 
Barytes is rather more abundant in the two satellite pipes examined 
than in the mountain. At a spot on the edge of the terrace east of the 
dyke G and south of the path there is much calcareous yellow weathered 
material which may mark the position of another subsidiary neck. 
It is extremely improbable that the present size and shape of the 
mountain bear any close approximation to what it was like when the great 
explosion produced it. The tuff and breccia wall projects over 1,000 feet 
above the highest remaining part of the Fish Eiver beds and the thick 
layers of clastic material dip everywhere towards the interior. It is 
obviously impossible that they could have accumulated in such a position 
only, they must have extended outwards in the form of a cone about the 
orifice, but only that part is preserved which came within the area of 
deposition enclosed by the original vent within the old crater. The 
material which accumulated outside the vent has long since been washed 
away, and it appears likely, from the distribution of the hardened rocks, 
that the process of hardening was produced by liquids or vapours rising 
approximately along the walls of the vent, so that a more or less tubular 
column of hard tuff and breccia and hardened sandstone resulted, en- 
closing loose rock and surrounded by a ring of loose rock. The total 
height of tuff and breccia exposed in section in the valley and the 
mountain above is about 1,700 feet. The enclosed loose rock is still 
preserved under the flat bottom of the central depression, but the similar 
material which dropped outside the vent has disappeared (see Fig. 1). 
On this explanation the depression in the mountain is not a crater, though 
it happens to be in the same position as the old crater, which was larger 
than it ; it is due entirely to the effects of erosion on a soft column of 
rock surrounded by a harder rock. The term caldera cannot be applied 
to it in any sense in which that term has been used by geologists who 
have described modern volcanos. The Spanish word caldera" is the name 
given to the more or less complete circular basin surrounding a crater, 
but there have been various attempts to give it strict definition and 
explanation ; according to one group of explanations it is a result of a 
violent explosion and subsequent partial filling in of the cavity produced, 
and according to another it was produced by the subsidence of a circular 
column of matter surrounding the crater. Professor Daly,''' who has 
recently summed up the question, decides to limit the meaning to depres- 
sions considerably larger than the vent itself, and concludes that they are 
due to explosion or the melting down of material by the igneous rock 
rising from the vent. In the case of Geitsi Gubib we can see the walls of 
the vent in section, and the hollow wdthin the mountain is much smaller 
* "Volcanic Eocks and their Origin," 1914, pp. 144-150. 
