High-level Gravels of the Cape. 47 



into blocks, which are rounded and become polished by the sand 

 blown over them. The fort protecting the village of Uniondale is 

 huilt on a very good example of wind-polished surface-quartzite 

 (Plate X. 2). The rock is an exceedingly good building stone : the 

 harder varieties can be used for foundations, bridge work, &c, while 

 the softer are useful for walling ; by choosing the particular hardness 

 varieties can be obtained which take mouldings and chisel work. 

 The softer kinds harden on exposure, and once hard are not subject 

 to rotting like the calcareous sands of the coast, as the cementing 

 material here is silica. It is interesting to notice that, with an 

 abundance of the best building stone in the immediate neighbour- 

 hood, the architect of the local Dutch Church, in picking the stone 

 for building the tower, took the Table Mountain Sandstone. The 

 rock in its normal condition is all right, but here it is penetrated 

 with shear-planes, along which white mica is developed ; the con- 

 sequence was that before the roof was in place the tower had to be 

 pulled down. In the gravel at the head of the Kammanassie Kiver, 

 beyond Uitvlugt, there are three distinct zones, which seems to point 

 to the fact that after the first layer of gravel was deposited the river 

 became more obstructed, and a higher layer was laid down, and sub- 

 sequently a third, which facts accord well with the theory that the 

 obstruction was due to the successive sinking of the land surface. 



The formation of the steep-sided gorges that commence at their 

 actual heads, with as great a height of wall as lower down, require 

 some explanation. No springs occur in them, and the erosion works 

 backwards as if an invisible steam-shovel were gnawing out the 

 material. Walther, in his monograph on the Deserts, makes out tha 

 these cul-de-sacs are peculiar to excessively dry regions subjected 

 now and again to tremendous cloud-bursts. Here, however, we have 

 them forming in a well- watered region, and though it is getting to be 

 a desert from the burning of the mountain veld, it was evidently in 

 former ages very much better watered than now. I think the ex- 

 planation may be got from studying the drainage system. The 

 Oliphant's Eiver has beheaded the Kammanassie Eiver and left 

 the present head in an aimless way out on the flats. To compensate 

 for this loss, the river has got a large supply of water from the Long 

 Kloof, which runs in at Uniondale, some eight miles down stream 

 from the nominal head ; the effective head now lying at the summit 

 of the Prince Alfred's Pass. Directly the main river got this incre- 

 ment, it was once again turned into a powerful chisel of erosion, and 

 rapidly deepened its course. The upper portion, though only con- 

 taining water after rain, was affected by this lowering of the general 

 level of the main valley, and erosion worked backwards by a kind of 



