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, 
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