66 Transactions of the South African Philosophical Society. 



guishable from the matted fibres of the ground mass. The large 

 olivine crystals are in all respects similar to those in the other 

 melilite-basalts. In all of these rocks the magnetite and perofskite 

 grains and crystals are frequently arranged round the olivine crystals 

 as a sort of border or frame. Some large masses of shale and grit 

 many feet in length are enclosed by the melilite-basalt of this dyke. 



The Sutherland Commonage vents, then, are of two kinds — one 

 variety is filled with melilite-basalt alone, or with that rock and tuff; 

 the second class is characterised by glassy and amygdaloidal lavas, 

 and contains some gritty tuff or agglomerate. There is nothing to 

 show which of the two classes was in activity first. The whole 

 group of vents is partially surrounded by a curved dyke of melilite- 

 basalt. 



The Matjes Fontein Pipes. 



On the farm Matjes Fontein, about 12 miles south-east from 

 Sutherland village, there are two very interesting pipes ; although 

 they are ill-exposed at the surface, shafts sunk by a prospector 

 enabled us to get a good idea of their nature. One of the pipes 

 is situated } mile south-south-east of the homestead, and the other 

 is at Silver Dam, a portion of the same farm. The two will be 

 referred to as the Matjes Fontein and Silver Dam pipes. 



The Matjes Fontein pipe has two pits sunk in it, one of which cuts 

 through the contact with the country rock, a greenish sandy shale 

 which dips at a high angle away from the pipe. This dip does not 

 extend far, as there are outcrops of sandstones and shales lying 

 nearly horizontally a few yards from the pipe. The size of the pipe 

 cannot be determined closely, but it is about 100 feet wide. Near 

 the contact with the green shale the contents of the pipe are an 

 agglomerate with a soft earthy matrix with much sandy matter in 

 it. The included fragments are mica plates up to an inch in dia- 

 meter, large lumps of ilmenite, irregularly shaped lustrous black 

 hornblende up to 3 inches in length, many pieces of shale and 

 sandstone derived from the Karroo formation, and large fragments 

 of granite and gneiss. The large fragments are angular or partially 

 rounded and they are scattered irregularly through the sandy matrix. 

 Near the second shaft there is a boulder of dolerite several feet in 

 diameter; its relation to the pipe is not absolutely certain, for it 

 projects from the soil and the surrounding rock is not laid bare. 

 There is, however, no doubt that it lies within the pipe, and as it 

 cannot be a boulder derived from dolerite now exposed at the surface, 

 for the nearest dolerite outcrop is more than a mile away, it is very 

 probably a block of dolerite lying in the agglomerate. In the second 



