Tlie Transkei Gap. 67 



America, and possibly in the case of the Beagle channel south of 

 Terra del Fuego, are due to the sinking of a long, narrow strip of 

 country between vertical faults ; thus, in one recently described in 

 Egypt by Dr. Hume, a narrow slip of cretaceous beds is faulted down 

 between granite cliffs that tower 1,500 feet above them on either 

 side.* In Eastern Sinai, where there is a number of these rifts 

 disposed in two diagonal series, in many of them there is no direct 

 evidence of dislocation, the nature of the valley being inferred from 

 their general appearance ; this is the case in several places along 

 the Transkei Gap where the rubble covers the underlying rock ; 

 and in such places it might very well be supposed that the valley,, 

 walled in by steep cliffs, running counter to the general drainage,, 

 might be due to the sinking of a narrow piece between faults ; on 

 following up, however, any particular valley, the true cause can 

 always be found in the presence of the coarse crystalline rock in 

 favourable positions. In the case of the Beagle Channel we have 

 not yet any accurate account from which we could assign its origin 

 to a Eift-valley or to a Gap-valley of the type we are describing ; 

 but from Darwin's description j of the geology of these parts, 

 which consists essentially of clay-slate, and metamorphic schists, 

 with intrusive granite and trap rocks, the Beagle Channel may well 

 l)e a dyke of crystalline rock which has been eaten into by the waters. 

 of the sea and the debris washed away. Darwin describes it in 

 the " Voyage of the Beagle " as about 120 miles long with an average 

 breadth of 2 miles: "it is throughout the greater part so perfectly 

 straight that the view, bounded on each side by a line of mountains, 

 gradually becomes indistinct in the long distance." This appearance 

 would be produced if the Transkei Gap were to be carried below sea- 

 level, the only difference, apart from its smaller size, being, that in 

 the South African Gap there are usually debris slopes on either side ; 

 but these of course would be removed if there were a powerful sea 

 running in the channel, and at the same time, owing to the under- 

 cutting of the sides, the gap would be widened. 



Our definition of the term " Gap-valley," therefore, will be " a 

 long depression between steep walls caused by the weathering 

 of a dyke," a somewhat unusual use of the term Gap, perhaps, 

 yet when seen in the country, the appearance of something miss- 

 ing, as though the earth had ** gaped" and opened, which was. 

 actually thought to be the case by the older land surveyors, makes, 

 the use of the term more appropriate. It is caused by the weather- 

 ing out of a dyke of diorite which yields more easily than the 



* " Kift Valleys of Eastern Sinai," (Abstract) Gcol. Mag., May, 1901, p. 198. 

 f C. Darwin, " Geological Observations," pt. ii. p. 151-156, 1857. 



