﻿Vol. 66.] ON THE GEOLOGY OF iNTASALAND. 207 



Middle Division. 



2. Shale Series, comprising black shales, grey grits, carbonaceous 

 shales, and coals. Thickness = about 170 feet. 



Lower Division. 



1. Lower Sandstones, under which head are included hard striped 

 flags, grey fine-grained grits and mudstones, and pebbly sandstone* 

 near the top. Thickness = about 1000 feet ; but the base is not 

 seen. 



In the south-west, as already stated, a boulder-conglomerate 

 occurs, which may probably be referred to the same horizon as the 

 purple sandstones. This conglomerate appears to be faulted out on 

 the north. As exposed in the Mwesia stream, close to its junction 

 with the Fwasa, it is found to consist of quartz and gneiss boulders, 

 occasionally measuring as much as 4 feet in length. These boulders 

 are subangular, and closely packed without assortment. 



The Mpata succession compares very well with the Karroo of the 

 Nkana area. The chief difference lies in the much greater thick- 

 ness of the calcareous group near Mpata. 



The only fossils as yet known in this district were found by 

 Drummond. These consist of fish-scales (Colobodus africanus y 

 Acrolepis ? drummondi, etc.) and lamellibranchs (Palceomutela ob- 

 longa) ; see Appendices II & III. They do not appear to afford 

 definite evidence as to what portion of the Karroo System it is to 

 which these beds belong. 



(C) Mwapo and Sere River Area. — If the Rukuru River be 

 traced up-stream from Mpata village, it is found to pass through a 

 narrow gorge in the gneiss hills. Above the gorge there is a sharp 

 right-angled bend in the course of the river, which is now found 

 flowing in a nearly south-to-north direction within a trough-shaped 

 valley. This valley is bounded on its eastern side by the Rukuru 

 Ridge, formed of gneiss, and on the west by a much higher mountain- 

 ridge known as Yirauli Mountain (fig. 5, p. 208). The Rukuru 

 Ridge drops steeply down to the bottom of the valley, and runs for 

 some miles in a remarkably straight north-and-south direction. 

 The Yirauli Ridge, on the other hand, is much shorter, terminating 

 abruptly on the south. The trough so defined is filled in with 

 Karroo Beds dipping usually in an easterly direction, at angles 

 steeper than the general slope of the valley-floor. At the southern 

 end of Yirauli Mountain the sedimentaries run back and up to 

 the west, and form a bay-like extension through which the Lower 

 Mwapo stream passes on its way to the Rukuru. The bay-like 

 extension is terminated on the south by a ridge of gneiss, which 

 runs at first south-eastwards and then turns more or less parallel 

 with the Rukuru Ridge, thus leaving a narrow trough filled with 

 Karroo rocks, but not occupied by the Rukuru, which has cut clean 

 through the ridge of gneiss on the west. 



A few miles away to the south-west, and across this ridge, lies a 

 remarkable basin-shaped area of Karroo surrounded, except on the 



Q. J. G. S. No. 262. p 



