Above the sea, and occupies the southern end of the great central rift-valley, which terminates suddenly at its southern point, the line of depression being represented farther south by the more easterly trough of Lakes Nyasa and Rukwa, from which Tanganyika is separated by the Fipa plateau, composed of old granitoid rocks; though even here traces of old valley-walls are said by Dr Kohlschiitter to exist.
~Great Granitoid Series (intru sive in the main, Laurentian).
Most of tie Archean rocks fall into one or the other of two great series, a schistose series and a granitoid series, the latter being in large part intrusive in the former.
The rocks of the granitoid series appear as great masses in the schist series, and in some places form great protruding bosses.
A remarkable feature in the volcanic phenomena was the disruption of the basaltic plateaus by large bosses of gabbro and of various granitoid rocks.