Tlie Volcanoes of Griqitaland East. 107 



creamy to brilliantly white. It forms thick massive beds often 60 to 

 100 feet thick, and weathers in extraordinary pepper-pot and mush- 

 room shapes, or again in immense tabular masses. It sometimes 

 •contains structures that look like plant stems, and cavities also in it 

 are filled in with chalcedonic silica, like in the agates and opals of 

 the lavas that lie upon it. It always accompanies the volcanoes, 

 and is especially thick in their neighbourhood, w4iereas in the east of 

 the district where the volcano line crosses over the Drakensberg 

 Mountains into Basutoland, the Cave Sandstone is absent for some 

 distance south of the Drakensberg, and the lavas rest directly on the 

 Eed Beds. The whole appearance of the rock is so strikingly 

 similar to a trachyte tuff, that I at first took it to be such, and 

 thought that it w^as the result of the first explosive outbursts of the 

 Tolcanoes, which were situated at that time beneath the sea, but 

 microscopic examination shows it to be made of quartz and felspar, 

 the latter largely microcline, but, except in one doubtful case, I have 

 found no volcanic glass, and no mica or other ferro-magnesium 

 constituent. Mr. Lewis, of the Government Analytical Laboratory, 

 has analysed it for the Geological Commission, and his analysis 

 finally settles that it is a sedimentary rock and not a tuff. 



The group of volcanoes just described taken together are arranged 

 along a line trending roughly 60° E. of N. ; if the western end is pro- 

 duced it will run through the three volcanoes discovered by Mr. 

 Dunn near Jamestown and Molteno. Whether there are volcanic 

 vents in between these tw^o groups, and whether they occur along 

 this line it is impossible to say until the survey of Barkly West 

 and Herschel is undertaken, but from the description of the country, 

 and from what we know of the occurrence of volcanoes elsewhere, I 

 think most geologists would say that there could be very little doubt 

 on the matter." If we accept the view that there is a whole series 

 of volcanoes arranged along a line of weakness in a direction about 

 60° E. of N., we have a weapon for attacking some very difficult 

 problems in the past history of South Africa — problems which have a 

 bearing on the structure of the continent as a whole. 



I am afraid this line of reasoning is somewhat complicated, not 

 that it involves obscure points, but as each step forms a problem in 

 itself, and as nothing has yet been published on the matter, each 

 step wants a great deal of discussion before it can be accepted as 

 proven. First of all it is essential to understand the river system 

 of South Africa. This consists of tw^o quite distinct series, an old 



* Mr. Dunn in a map made to accompany the same report, G4, 1878, but 

 never published, shows two more volcanoes to the east of Jamestown, namely, 

 Olat Kopjes and Wonderbosch Klip. 



