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Eaasay and in the Slieve GuUion area in Ireland, where they are in a 
fissure clearly associated with andesite and rhyolite/'' 
In such cases as those from Scotland it is supposed that the explosive 
force which opened the vents and filled them with fragments of the rocks 
surrounding the vents was directly connected with the magma which gave 
rise to other and more normal volcanic phenomena in the same neighbour- 
hood, but that locally the force became exhausted in the effort before 
fragments of the magma itself could reach the surface. In the case of 
Geitsi Gubib, which is certainly a very large neck, it is difficult to under- 
stand how the results of so great an explosion could be deficient in fragments 
of volcanic rocks. It may be that such fragments are in the tuffs there 
in a minutely subdivided and unrecognizable form, or that they are to be 
found at a lower level than the present outcrops. 
I saw no igneous rocks in the immediate neighbourhood of Geitsi 
Gubib, but pipes of blue-ground exist not far off, near Gibeon, and to the 
west and north-east of the mountain,! and Karroo dolerites occur 20 miles 
to the south-east at Baviaans Kranz. 
As to the age of the mountain there is no satisfactory evidence as yet. 
The Karroo beds, so far as we know, do not lie close enough to the moun- 
tain to be affected by the outward dip of the strata in its immediate 
neighbourhood, but the road from Dairacharal Drift to Tses crosses 
a breccia dyke in those beds, and it is likely that this dyke was produced 
at the same time as Geitsi Gubib. The presence of fragments of Karroo 
beds in the breccia of the mountains is possible, but cannot be said to 
be proved, and there is no reason for suspecting that the Karroo beds on 
its site were thick at the time of the outburst. There are terraces 
at various levels round the mountain from which much information 
about its previous history will certainly be got, but my observations were 
too scanty and hurried to form the basis of a discussion. From the 
height of the upper terraces and their proximity to the neck, however, 
it seemed that the mountain need not have been very much higher than 
it is now when the outflowing stream began to cut through the Fish Eiver 
beds at the south side of the neck. 
* Sir A. Geikie, " Ancient Volcanoes," ii., pp. 423-4. 
f Eighteen of them are marked in Dr. Kange's map, and he states that over thirty are 
known, "Geologic des Deutschen Namalandes " ; a brief description is given by 
Dr. Scheibe in " Der Blue-ground," Prog. d. Konigl. Berg. Akademie zu Berlin, 1906. 
