Tlie Transkci Gaj?. 73' 



though at right angles to the Gap, shows a similarity in the nature 

 of the crack in which it came to its present position. The outcrop 

 is about '100 yards wide and about a mile long ; on either side there 

 are Karroo rocks and included dolerite sheets lying undisturbed, 

 l:»oth of wdiich have been pierced by the granophyre. It does not 

 form a valley, but stands up more like a granite boss and the surface 

 is weathered into large boulders. A tine section through the mass is 

 afforded by a deep ravine that cuts through it, but the actual 

 junction of the granite with the other rocks is obscured by debris. 



The above description can only be regarded as a preliminary 

 note, as there are still several points wdiich further work only 

 can make clear, such as the nature of the western termination,, 

 but as it will be some time before the survey can be carried on in 

 those parts, the occurrences seemed of sufficient interest to give the 

 Society a description of it, from the standpoint of our present 

 knowledge. 



The rock forming the dykes of the Gap is a peculiar one, differing" 

 in important respects from any intrusions hitherto found by us in 

 the Karroo Series, although, as will be pointed out in the following 

 notes, it has a distinct relationship to the olivine-dolerite of the 

 sheets." It consists chiefly of the following minerals in the order of 

 their usual relative abundance : — plagioclase, hornblende, augite, 

 quartz, red-brown mica, orthoclase, apatite, iron ores, sphene and 

 decomposition products such as chlorite, uralite, and calcite. Varia- 

 tions in the proportions of these minerals show that the rock differs 

 considerably in composition from point to point. 



The plagioclase has almost always a zonal structure ; the extinc- 

 tion angle of the innermost part of any particular section is in 

 general much higher than that of the outer, which indicates that the 

 inner part of the crystal is of more basic composition than the outer. 

 The plagioclase frequently shows crystal outlines when in contact 

 wdth the hornblende and augite, sometimes small crystals of the 

 felspar are entirely enclosed by tlie hornblende and augite. This- 

 ophitic structure, though found without difficulty in all the slices of 

 the rock examined, is not nearly so pronounced a feature as in the 

 olivine-dolerite. 



The original hornblende is mostly of a pale greenish-brown colour, 

 with feeble plechroism, but a bright green strongly pleochroic variety 



* The olivine-dolerite which forms the intrusive sheets of the Transkei is very 

 like the rocks occurring in the same manner near Beaufort West, and described 

 by E. Cohen, Ncu. Jahrb. f. Min., 1874, p. 195. 



