THE TANGANYIKA PROBLEM. 65 



In the Nyassa region, it is thus seen that there are three 

 very distinct types of aqueous deposit. Beginning at the 

 top of the series, there is the undisturbed and obviously 

 modern lake deposit, then the slightly disturbed and tilted 

 white chalky limestones found bordering Nyassa itself 

 (pleistocene), and beneath these are first Drummond's beds 

 (probably triassic estuarine), and then the much older, ex- 

 tensive and probably marine sandstones of the Mount 

 Waller series. All these three deposits are quite uncon- 

 formable one with another, and they represent quite distinct 

 periods in the geological history of the region in which they 

 occur. But since they are found to appear extensively 

 elsewhere, and will be encountered again and again as 

 we proceed, it will be convenient to distinguish the periods 

 to which they belong by distinctive names, and I shall 

 therefore in the sequel speak of the lowest as the Old 

 African sandstones, the next as Drummond's beds, and the 

 white limestones of Nyassa as the African-lake-pleistocenes. 

 Drummond's beds, so far as I have encountered them, 

 are never very extensive, they only cover a few miles at the 

 north of Nyassa, but the Old African sandstones, on the 

 other hand, are not only in places more than three thousand 

 feet thick, but cover at the present time actually millions of 

 square miles. These Old African sandstones, so far as their 

 examination has gone, have nowhere yet proved fossiliferous ; 

 but there is, as I have said, a close similarity between the 

 massive sandstones of Mount Waller and the massive sand- 

 stones which flank the east and west of Lake Tanganyika, 

 for a hundred miles from the south. All these more 

 northerly sandstone deposits appear further to be similar 

 to, and actually continuous with, the formations occurring 

 to the west, in the regions of Lake Mwero, and to 

 the east of Tanganyika, throughout the region of Rukwa, 



5 



