THE TANGANYIKA PROBLEM. 
7i 
it would appear probable that the lake must have continued 
to extend northwards as the fold increased, and this view of 
the matter seems to be supported by the very singular fact 
that, as we found, in several places off the Livingstone 
range, the bottom of the lake although the water was 270 
fathoms deep, is composed of practically bare rock, and 
quartz fragments, whereas further south the whole floor of 
Nyassa is covered with fine deep mud, or, in other words, 
it is suggested that the origin of this portion of the lake’s 
floor is so modern that up to the present time very little 
fine sediment has had time to collect upon it. Similar 
evidence is afforded by the upraised lake deposits in the 
neighbourhood of Mount W aller. These deposits are not 
more than 70 to 100 feet thick, they do not appear to be 
much denuded, and consequently they cannot have been 
deposited for any great length of time. In the south, on 
the other hand, we have in the region of Fort Johnston and 
Pamalombi, obviously vast depressions which are filled with 
lake deposits, and these latter, on the very lowest estimates, 
must be many hundreds of feet thick. It would appear 
therefore that the old fold which originally contained the 
southern portion of Nyassa has progressively extended 
northward, and, consequently, we encounter here for the first 
time, a fact which we shall encounter again, namely, that 
different portions of the same Lake valley may be of 
very different ages. 
We saw that towards the north end of Nyassa the old 
African sandstones and Drummond’s beds were both much 
contorted and often sloped at a high angle out of the floor of 
the present lake. There is no conformity between these 
series, and there must consequently have been in this area, 
first a period when the old African sandstones were laid 
down in deep water, afterwards there was a rising of the 
