﻿220 MESSES. A. E. ANDEEW AND T. E. G. BAILEY [May I9IO, 



similar, though thinner, fault-veins occur far to the north, in the 

 Mwapo and Western Nyika areas. The Shale Group consists of 

 flags, mudstones, and shales, with layers of ironstone, the shales 

 containing leaves of Glossopteris. 



(4) Upper Sandstone Group. — This group is much obscured by 

 alluvium. The sandstones follow conformably upon the underlying 

 shales, and dip at first westwards. Some 3 miles out the dip turns 

 over to the east, and continues so for some distance. Partner on, 

 however, the dip again turns over, and with this- dip the sandstones 

 pass under the lavas of the upper group. 



The sandstones and grits are, as usual, thickly bedded and pebbly 

 in places. The whole group has a thickness of some 7000 feet. 



(5) Lava Group. — The lavas dip steadily westwards to near 

 Kasembe Hill, where the dip changes to east. They consist for 

 the most part of vesicular basalt, while some appear to be andesites, 

 and banded rhyolite has been observed, though not in situ. The 

 vesicles are usually filled with chalcedony or more rarely calcite. 

 Agates are often plentifully scattered over the weathered surface 

 of the lavas. The individual beds appear to be of considerable 

 thickness. No tuffs or laterites are found between the flows. 

 About 1000 feet of lavas are exposed in almost vertical section at 

 Murukunyama Hill. 



Near the base of the Lava Group there is at least one inter- 

 bedded stratum of sandstone, and similar sandstones are found above 

 the lavas in Portuguese territory. Below Myowe Hill, close to 

 the Mwanza Piver, a narrow strip of amygdaloidal basalt is found 

 faulted against the gneiss on the east and the Lower Sandstones on 

 the west. The eastern fault-line is marked by a great quartz- 

 reef occasionally brecciated : this forms the Myowe ridge. 



Sills, dykes, and laccolites in the Karroo Series of 

 the Lower Shire District. — Numerous sills of fine-grained 

 basalt, or more rarely dolerite, are found intrusive into the 

 members of the shale-group, near Sumbu and Zimbawe. Similar 

 sills are also observed in the shales of the Nachipere district. 

 Dykes or sills appear to be very rare in both the Upper and the 

 Lower Sandstones. It is obvious that the distribution of the sills 

 was determined by the non-resistant qualities of the soft shales. 



Irregular laccolitic intrusions of dolerite, however, occur in the 

 upper sandstones, as near Nkombedzi villages. Although these 

 dolerites are obviously intrusive, they are nevertheless amygdaloidal 

 in character. It seems probable that the intrusion of dolerite was 

 followed closely by minor earth-movements, which not only cut off 

 the laccolite from its parent magma, but released the pressure on 

 the intrusion, so allowing the occluded vapours to escape. 



Very occasional quartz-porphyry dykes have been noticed cutting 

 the lavas themselves. 



