68 Transactions of the Soutli African PhilosopJiical Society. 



compact horizontal shales and sandstones of the Karroo Series into 

 which it has been intruded ; on a small scale, the same phenomenon 

 •can be seen on any sea coast where basic dykes are intruded into 

 harder rocks. 



Actual mapping of the Transkei Gap was only carried out in the 

 Kentani Division where it runs in an almost direct east and west 

 line for a distance of 36 miles. It enters the division a little north 

 of the N'Debe Eiver where it debouches into the Gcua or Butter- 

 worth Eiver ; it is here an ordinary dyke of diorite weathering out 

 ■on the surface of the ground in large boulders, 3 to 6 feet in 

 ■diameter. It can be followed across the bed of the Gcua, east- 

 wards up a short ravine ; on looking eastwards from the low nek 

 ,at the head of the ravine one sees the typical Gap-valley in which 

 the course of the N'Debe Eiver runs. The top of the cliff" above the 

 Gcua and N'Debe is at the general level of the hills and ridges in 

 the neighbourhood and is, in fact, part of the plateau which is 

 the dominant feature of the country. Into this plateau the rivers 

 have cut deep and precipitous-sided valleys winding in and out in a 

 manner one would expect them to do on a fairly level plain, and the 

 original plateau-form is often obscured. These facts show us 

 clearly the course of events which have taken place in the recent 

 geological history of this region. The plateau was a plain of river 

 erosion or a peneplain, near sea-level, and the rivers, having long 

 courses over ground which gave them a very slight fall, wound 

 about on the surface, constantly changing their beds, heaping up 

 banks of sand and gravel at one place, and removing previously 

 deposited material from another. Then the land gradually rose and 

 immediately the rivers began to erode downwards, instead of 

 sideways ; the rising of the land has been so continuous that the 

 rivers have not had an opportunity of straightening their courses, 

 but still run in the tortuous channels in which they were running 

 before the land movement took place. 



The same elevation has taken place all along the south-east coast 

 of South Africa, from Hang Klip to Natal, but in the west the 

 average elevation of the coastal plain is less than in the country 

 under discussion. The river-gravels were mostly washed away 

 when the land rose and degradation set in with increased intensity ; 

 nevertheless, there are always patches, often considerable in the 

 west, which have escaped and remain perched on the top of the 

 ridges ; such a patch of river-deposit is to be found on Kentani Hill 

 south of the two Gap-valleys, and owing to its exposed position and 

 the action of water, dissolving the silica of the grains, then depositing 

 it again in the interstices on evaporation, the sandy parts have 



