Dr. John Wm. Evans — Recent Breccias in Bolivia. 549 



impossible state of afiairs in the absence of earth-movements for 

 long-continued periods. 



The condition of things just remarked upon brings us naturally 

 to the second point — 'rift-valleys,' which have been brought forward 

 to explain the features so obviously due to denudation having been 

 ■confined to the stream-beds, with minor complications in the 

 way of lake deposits and uplift or subsidence. It is the latter 

 details to which the theory is primarily due, as it never seems 

 to have occurred to anybody that slight movements of elevation 

 or depression may cause deposition of sediments in a deep valley. 

 The Zambesi due north of Bulawayo flows in a broad valley over 

 a floor of Tertiary sandstones, etc., about 1,500 feet above sea-level, 

 with Archasan schists and granite rising into hills over 4,000 feet 

 high within a few miles. But nearer its mouth it crosses an axis 

 of elevation which has given rise to the Shire highlands and the 

 mountains of the Rhodesian border, as well as the Drakensberg and 

 other ranges which have forced the rivers further south, like the 

 •Orange, to flow from close to the east coast right across the continent 

 to the western sea. Is it not reasonable, therefore, to suppose that 

 the Zambesi, after eroding a deep valley, was foi'ced by elevation 

 near the coast to deposit its sediments in a great lake which at 

 first covered a wide area of country ? Erosion would to some extent 

 keep pace with denudation, and as the uplift was not sufficient at 

 its maximum to reverse the course of the river, when it ceased the 

 lake would first be confined to the original wide valley of erosion, 

 and finally the river, as it is doing to-day, would reduce the level 

 of its bed until it made a new one in the sediments it had itself 

 previously deposited.^ 



No one would now argue that the Colorado canons or the valleys 

 of the Blue Mountains are due to anything but normal erosion; 

 why, then, should our unfortunate African rivers be still given over 

 to the catastrophist ? 



1 



VIII. — Eecent Breccias in Bolivia. 



By John William Evans, Ll.B., D.Sc, F.G.S. 



N February of last year Professor Bonney read an instructive 

 paper before the Geological Society," in which he enumerated 

 and briefly described a number of the principal breccias found in 

 the geological series. He also gave a short description of some of 

 the best known recent breccias — the stone rivers of the Falkland 

 Islands and the angular detritus that fringes the foot of the hillsides 

 in Persia, Central Asia, and Northern Hindustan, In conclusion, 

 he expressed his opinion that most of the ancient breccias referred 

 to in his paper indicated " a climate, arid, liable to extremes of 

 temperature, with cold winters. The precipitation probably was 



1 In the case of undoubtedly marine sediments like those of Egypt, rapid sub- 

 mergence and subsequent re-elevation would account for the phenomena observed. 



- Q.J.G.S., Iviii, pp. 185-203 ; see also a paper by Professor Blake in the same 

 volume, p. 290. 



