70 Transactions of the South African Philosophical Society. 



three-quarters of a mile long; beyond it to the east there is a typical 

 east and west valley, the head of the N'Kukuwana Eiver, in which, 

 however, no crystalline rock was found. East of this there is no 

 evidence of either a Gap-valley or of any Gap-rock till we come to 

 the Cat's Pass, the reason for this probably being the presence of a 

 thick sheet of dolerite, a northward extension of the great Kologha 

 sheet, which here covers the plateau and has apparently proved an 

 insuperable barrier to the upward rending of the rocks by means of 

 which the Gap-rock ascended. From the Cat's Pass there is a long, 

 sharp ridge, almost a knife-edge, running east and west and bordered 

 on the north and south by deeply-cut ravines. Along this ridge the 

 main road from Butterworth to Manubi forest is carried for a 

 distance of about 4 miles. At the Cat's Pass the diorite is seen on 

 the northern side of the ridge somewhat far down, but it presently, 

 as one goes eastwards, comes on top of the ridge and is practically 

 the cause of the existence of the ridge — that is to say, the harden- 

 ing that the sedimentary rocks have undergone, owing to the in- 

 trusion of the mass of molten rock, has been sufficient to render 

 them more resistant to weathering than the rocks further away 

 from the Gap-rock ; the nature of the latter here is in no wise 

 different from that in places where it forms the typical Gap. From 

 the point where the main road to Manubi turns sharply south by 

 Lusizi, the Gap-rock can be followed in isolated outcrops on the 

 level plateau which exists here, each outcrop being linearly east of 

 the last, until we get within 4 miles of the Kogha River. From 

 here there is again a typical Gap-valley thickly wooded on the north 

 side. The whole length of this part of the Gap is occupied by a 

 single straight stream which eventually joins the Kogha, and a corre- 

 sponding valley is seen on the other side of that river stretching away 

 into the Willowvale Division. In the latter part of the course of this 

 gap the direction is a little north of east. About 2 miles from its 

 junction with the Kogha a long, straight valley is seen trending 

 eastwards in an east-north-east direction ; it crosses the Gap, and is 

 seen to be continued over the other side of the Kogha running in a 

 similar direction ; no crystalline rock was found in this valley ; it 

 has, however, all the appearance of a Gap- valley and may owe its 

 origin to the same cause. 



Turning now to the southern branch, a very line view can be 

 obtained on the Kentani main road a little south of Gobogobo ; to 

 the west a succession of steep valleys running in a straight line and 

 separated by low neks are seen, but no crystalline rock can be found 

 in them until one reaches the crossing of the two branches of the 

 dykes. On the east one looks down a long, straight valley, the 



