THE INTRUSIVE DOLERITES AND ALLIED ROCKS 271 



into by the stream to a depth of 300 feet, yet the lower 

 surface is not exposed ; it thins out in this direction very 

 rapidly and disappears. 



The upper half of the Kologha Valley lies in an exten- 

 sive sheet which is continued across the Kei in the 

 Komgha Division. The cliffs and slopes on the left 

 bank of the Kei for a distance of four miles rise some 

 1,200 feet above the river, and two-thirds of the vertical 

 height are composed of the dolerite of the Kologha sheet. 

 On Inver Gcua the sill dips northwards across the sedi- 

 mentary rocks, while to the east it gives off thin sheets. 



The thickness of the intrusion varies greatly ; on the 

 Kei it is as much as 900 feet thick near Mimosa Dale, 

 where both top and bottom are seen in the cliffs. On 

 the Kobonqaba Kiver it is at least 500 feet thick, but 

 about five miles to the north-east it thins out com- 

 pletely. 



In Pondoland and East Griqualand there are some 

 very large masses of dolerite, much thicker in proportion 

 to their area than any of the sheets hitherto mentioned. 

 The Tsala hills near Lusikisiki are small examples of 

 these masses, and larger ones are N'Tabankulu, Insizwa, 

 Mount Ayliff, Mount Currie, and the Ingeli Mountain. 

 These seem to be thick lenticular or cake-shaded bodies 

 of rock, but their structure is not known in detail. The 

 sedimentary rocks near them do not appear to be dis- 

 turbed appreciably. 



In the northern part of Cape Colony the dolerites are 

 still abundant, but the country is much flatter and they 

 do not give rise to such great ridges as farther south. 

 The dolerite outliers in Kimberley, Barkly West, Her- 



