PHYSICAL GEOGRAPHY OF THE CAPE COLONY. 255 



dome, after the fashion of the laccolites of the Henry Mountains 

 described by Mr. Gilbert. The planes of division produced by con- 

 traction would then naturally run parallel to the curved upper surface. 



Besides sheets and dykes there are also intrusive masses of trap, 

 some of which cover areas of many square miles and form great 

 hill-groups. One very large mass forms the hills known as the 

 Andries-Bergcn, about 18 miles N.N.W. of Queenstown *. The 

 rock is very similar in appearance to that of the sheets and dykes, 

 only on the whole somewhat coarser in grain. It consists of laths 

 of plagioclose ('2 to '6 millim. across), augite, olivine, and ilmenite. 

 The plagioclase penetrates both the augite and the olivine ; the 

 olivine is in large crystals. The rock bears a very close resemblance 

 to that of the intrusive sheets of Beaufort West and Colesberg ; the 

 main points of difference are that the felspar-crystals are larger and 

 there is more olivine. The coarser grain may very reasonably be 

 attributed to a slower rate of cooling due to the larger bulk of the 

 mass. 



I also brought away specimens from a small intrusive mass close 

 to Burgersdorp. The specimen from the centre of the mass is very 

 similar to the rock of the intrusive sheets of the top of the Hangklip 

 and Dordrecht in its microscopical character ; there is rather a 

 larger proportion of plagioclase and the crystals are larger in size, 

 •2 to '3 millim. across. A specimen taken close to the edge of this 

 mass is much more closely grained than the preceding. Under the 

 microscope it is seen to be a finely felted mass of small felspar- 

 microliths, and grains, probably of augite, in a black opaque ground- 

 mass. Scattered about are larger crystals of plagioclase, up to '6 

 millim. across, shattered and broken, plates of augite up to •! millim. 

 across, and blebs of what seems altered olivine. 



I have already mentioned that I saw the subaerial lavas, which 

 form the highest subdivision of the Stormberg Beds, only on Yaal 

 Kop. The three specimens I brought away from there have the 

 following characters. They are more closely grained than the 

 intrusive traps and decidedly vesicular or amygdaloidal. No. 1 is 

 very similar to the rock of the sheet on the top of Hangklip. 'No. 2 

 contains many laths of plagioclase -04 millim. across on an average ; 

 the space between these is filled in by viridite and small transparent 

 granules, some of which appeared to be augite and some possibly 

 olivine. A few larger crystals of plagioclase, shattered and corroded, 

 are scattered about. It is very like the thick sheet in Kimberley 

 Mine, but the plagioclase laths are smaller. No, 3 is very like No. 2, 

 only the plagioclase laths are smaller and the other constituents 

 more decomposed and more difficult to identify. It contained large 

 fragmentary crystals of plagioclase, augite, and olivine. 



So far as the evidence goes, the resemblance between the lava- 

 flows and the intrusive igneous rocks is fairly close, so that both 

 may well have come from the same source. 



* The large and prosperous farm of Carnarvon, belonging to Mr. Halse, 

 stands just on the edge of this mass, and I recollect with pleasure the hospitable 

 welcome we receiyecl there. 



