Correspondence. '89 



undergone, then, is probably due to that series of causes comprised 

 in the term subaerial, and into which the action of sun and wind, 

 rain, ice, and the erosion by river and streams, enter as principals. 

 I will proceed briefly to describe the amount of this denudation, 

 and to notice some facts bearing upon its cause, more especially 

 with reference to the action of rivers and the formation of their 

 valleys. 



It has been stated that these "Lacustrine strata " reach the sea at the 

 Qualana mouth ; and, as everywhere they are nearly horizontal, and 

 we have at no place been able to discover faults of any considerable 

 extent, the elevation of the higher ranges of mountains will give 

 the thickness of the formation. This, unfortunately, we can only 

 give approximately, as there is no reliable^ survey of the Quathlamba 

 Eange that I am acquainted with. Where I saw the continuation of 

 it, the Wittebergen, it is about 3,000 feet above the elevation of the 

 great plateaux among which the Kei and Tsomo take their rise ; 

 but I believe the more northern parts reach a still greater elevation. 

 The Wittebergen are 6,000 to 7,000 feet above the sea : the highest 

 point of the Quathlamba is said to be 10,000 feet. The plain 

 through which the Orange and Caledon Elvers run is about 5,000 

 feet above the sea ; and a series of plains, of various elevations, 

 interrupted by mountains, reach thence to the sea on the north, and 

 to the Karoo plains, bounded by slopes of the Zwartebergen and 

 Zuurbergen. The heights of these plains above the sea is probably, 

 on an average, about 1,200 to 1,500 feet, and they vary from 30 to 

 20 miles in breadth. The whole of this denudation must have been 

 due to subaerial agents ; and that the large periodical torrents must 

 have borne the chief part in it, the following facts will, I think, 

 show. The Orange, the Caledon, and the Krasi Elvers, traversing 

 the elevated plains, have all their sources in the same ranges, whose 

 geological structure is but imperfectly known at present ; but it 

 is certain that a large mass of amygdaloid, containing agates, 

 cornelian, and other silicious minerals, forms part of it. The rivers 

 carry numbers of pebbles of amygdaloid and its agates, and as these 

 are wholly unlike the rocks and minerals of the country they 

 traverse, it is easy to find their traces ; and these I have seen on the 

 plains and hills many miles (certainly five or six) distant from, and 

 300 to 500 feet above, their present course. Now these minerals, 

 though comparatively imperishable, must be subject to decay, and to 

 be washed away by the heavy rains of these regions, so that, 

 when found in the sites indicated, they must prove that amount of 

 denudation by the river within a recent date. The manner in 

 which these rivers travel over the plains is shown by the presence 

 of the Iridina, which inhabits them in all parts. This is the 

 case, for instance, in Graff Eeinett. In whatever part of the plain a 

 well is sunk, the river-alluvium and pebbles, with the Iridina, will 

 be found. The courses of the valleys in which the rivers flow show, 

 as I think, incontestibly, that they have been made by the rivers 

 themselves. Those which run down the steep mountain- sides of the 

 Katriverberg have an extraordinarily sinuous course. A stream, two 



