200 ME. G. W. LAMPLTJGH ON THE [May I907, 



solution and redeposition of silica, as Dr. Passarge has admirably 

 shown, has been throughout their history one of the most character- 

 istic geological processes within the great sand-velds of south-central 

 Africa. The ' silcretes ' of the region which I traversed appear in 

 all respects to resemble the surface-quartzites of Cape Colony- 

 described by Mr. A. "W. Rogers * and his colleagues ; and, as with 

 these, the conditions for their production still prevail. Therefore, 

 while the induration of some may be of considerable antiquity, that 

 of others is certainly very recent. 



From his study of similar deposits in the Kalahari Desert, where 

 they attain their widest development, Dr. Passarge believes that a 

 definite sequence of events may be traced in them, involving many 

 changes of climatic conditions and dating back from Eocene times. 

 Hence he divides these deposits into several stages, which are grouped 

 together as a system under the term Botletle-Schichten. 

 From the descriptions of travellers he has concluded that these 

 i Botletle-Schichten ' are exposed with considerable regularity along 

 the dissected edges of the plateau in the Zambezi basin below the 

 Falls ; and he has therefore indicated their presence in this region 

 on his map. 2 



However, as will be gathered from the foregoing account, I do 

 not think that the suriace-deposits of the Batoka country afford 

 support to the complicated hypothetical deductions which Dr. 

 Passarge has drawn from the ' Botletle-Schichten ' of the Kalahari. 3 



In the country between Bulawayo and the Zambezi, Mr. Molyneux 

 describes the occurrence of ' innumerable heaps of travertine,' com- 

 posed of ' rounded quartz-grains cemented together by silica, often 

 assuming the shapes of roots,' which form 4 the capping of small 

 rises/ 4 These he considers to have been deposited by hot springs 

 now extinct ; but I venture to suggest that in most cases they are 

 surface-quartzites similar to those described above. 



The petrographical characters of two of the surface-quartzites are 

 described by Mr. Thomas in Appendix I [F 1045 & 1046] p. 211. 



Blocks of the chalcedonic quartzite are among the commonest 

 pebbles of all the streams that trench the ground which it occupies. 

 The stone has been used in ancient times by the natives for 

 manufacture into cutting implements. These implements lie 

 scattered in large numbers on the rocky flats bordering the upper 

 part of the Batoka Gorge : and, as I have elsewhere shown, 5 some 

 may be of considerable antiquity. 



1 ' An Introduction to the Geology of Cape Colony ' London, 1905, 

 pp. 357-61. 



2 ' Die Kalahari ' 1904, pp, 540-42, & Kartenband, Blatt ii. 



3 Ibid. p. 648. 



4 ' The Sedimentary Deposits of Southern Rhodesia' Quart. Journ. Geol. Soc. 

 vol. lix (1903) p. 282. 



5 ' Notes on the Occurrence of Stone Implements in the Valley of the 

 Zambesi around Victoria Falls ' Journ. Anthrop. Instit. vol. xxxvi (1906) 

 pp. 159-69. 



