THE DESICCATION OF AFRICA. 187 



Dr. p. A. Wagner, B.Sc. : The thanks of the Association 

 are due to the author for having brought this very important 

 matter before us. We, the colleagues of Professor Schwarz, do 

 not always see eye to eye with him in geological matters, but we 

 know that he can be relied upon to present his view^s in a most 

 interesting manner, and you will, I think, agree with me that on 

 the present occasion he has excelled himselif. 



Many of the questions raised in the paper have already been 

 fully discussed. I shall therefore confine myself, as far as 

 possible, to points not hitherto dealt with. 



In the first place, I should like to offer a word of criticism in 

 regard to the author's conception of the Proto-Orange. Professor 

 Schwarz joins up to Loangwe River with the lower course of 

 the Molopo, and thus conjures up the vision of a mighty stream 

 traversing what is at present the Kalahari. Now there can be 

 no question that the Molopo at soine former period must — at 

 times, at any rate — ^have been a swiftly-flowing stream, for, 

 during a recent visit to the Southern Kalahari, I found on a 

 small island in the middle of the present dry channel, about six 

 miles south of the Upington-Keetmanshoop railway, an extensive 

 terrace of coarse river wash, 15 feet in thickness, with well- 

 rounded boulders up to 18 inches in diameter. lu regard to the 

 age df the deposit, the top of which is about 35 feet above the 

 river-bed, there is no evidence. It may, however, be, and in all 

 probability is, very ancient. The Molopo Valley itself is without 

 doubt of great antiquity, and, like some of the old valleys leading 

 down to the Orange River from the south, was probably in 

 existence in Cretaceous times. The Loangwe, on the other 

 hand, flows in a rift-valley of comparatively recent origin. 

 Indeed the faulting to which the valley owes its origin is still 



cause of precipitation, and that because of their presence the precipitation 

 of the Lake country is greater than it would be were they absent. 



" The increase in precipitation due to the presence of the Great Lakes 

 is believed to be small, probably not more than two or three inches annu- 

 ally. This conclusion is reached by considering the precipitation of the 

 lake region in its relation to that of the remainder of the country east 

 of the Mississippi. The heaviest precipitation of the Mississippi occurs 

 in the Southern and South-Eastern States, and the amount diminishes 

 both in a northerly direction toward the interior of the continent and in 

 a westerly direction toward the sub-arid region of the far West. The 

 rate of decrease of precipitation, considering the region of 50 inches as a 

 reference point, is quite sharp in a westerly direction, being as much as 

 30 inches in 450 miles, or an inch in 15 miles. In a northerly direction 

 the decrease is much less ; in 300 miles it is at the rate of i inch in .30 

 miles ; for the next step northward, which includes the lake region, it is 

 only an inch in 50 miles. In other words, the rate of diminution between 

 the isohyetal of 50 inches and the isohyetal of 40 inches, if projected 

 northward, would cause the isnhvetal of 30 inches in the lake region 

 to pass about 200 miles southward of its present position. This displace- 

 ment would mean an average loss of two or three inches of precipitation 

 over the greater portion of the lake region. Considering also the distribu- 

 tion of precipitation on both sides of the lake region, it would appear 

 that the foregoing estimate is a fair allowance for the increase of precipi- 

 tation which might reasonably be attrilnited to the effect of the Lakes 

 themselves." 



