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THE TEANSKEI GAP. 



By a. W. Eogees and E. H. L. Schwarz. 



(Bead November 27, 1901.) 



One of the most interesting features of the Transkei, in which 

 territory the Geological Survey has lately been working, is a gap or 

 trough that runs east and west through the country. It is locally 

 known as the Transkei Gap and is produced by a vertical dyke of an 

 easily weathering crystalline rock allied to diorite, which has been 

 intruded into the sedimentary rocks of the district. The sedimentary 

 rocks consist of Karroo Beds lying nearly horizontally ; they had 

 already been injected by sheets and dykes of dolerite before this 

 later intrusion took place, so that the newer dyke cuts across both 

 the sedimentary rocks and the dolerite. The Gap is reduplicated in 

 several places, and there is an off-shoot at right angles to the general 

 direction. We have observed the Gap from Toleni Bridge to the 

 place where it runs into the sea at the mouth of the Kogha Kiver, a 

 distance of 50 miles, but we have more or less reliable information 

 that the Gap continues across the Kei into the Colony, as far as 

 parts of the Cathcart Division, which would make the whole length 

 of the dyke to be over 100 miles. 



The term " Gap" for a valley of this kind is of local origin and is 

 found on most of the older maps, such as *' A Plan of the Territories 

 formerly known as Kaffraria Proper, Cape Town, 1884 ; " still older 

 maps have a line drawn, showing the course of the Gap, with a note 

 explaining that the Gap " was probably formed by an earthquake." 

 Mr. McKay in the Transactions of this Society for 1884 (Cape Town, 

 1887) described the Gap and gives the proper explanation of the 

 occurrence, but he did not see that this dyke was peculiar, and had 

 important differences which distinguish it from, the ordinary dykes 

 of the neighbourhood. 



It is of the first importance to distinguish such a trough, due to 

 the weathering out of a soft dyke, from a '* Kift-valley," which 

 produces a somewhat similar type of surface feature. Eift-valleys 

 such as occur in Central Africa, Egypt, the western States of 



