﻿Yol. 66.~] ON THE GEOLOGY OF NYASALAND. 217 



thickness. In Southern Nyasaland the Karroo occupies about 

 800 square miles of low-lying country covered with stunted 

 thorn-trees, and is exposed along the south-eastern border of the 

 Protectorate, roughly between lat. 16° and 17° S. The Karroo is 

 bounded on the east by the gneissic foot-hills of the Neno Plateau, 

 then by the wide alluvial plain of the Lower Shire Eiver, and, 

 finally, by the crystalline schists and gneisses of the Port Herald 

 Hills. The junction between the gneiss and the sedimentary rocks 

 appears in all cases to be a faulted one. The western boundary of 

 the Karroo is purely arbitrary, being formed by the Anglo-Portuguese 

 boundary-line. The Karroo, as a matter of fact, crosses the border, 

 and stretches without interruption to the Zambezi River. It is 

 divisible into the following six groups, in descending order : — 



Approximate thickness in feet. 



6. An uppermost division of pebbly sandstones, found only in 



Portuguese territory near Sinjal (at least) 200 



5. A Lava Group containing a few interbedded sandstones... 5000 



4. An Upper Sandstone or Grit Group, consisting of a great 

 thickness of massive sandstones or grits, in beds measur- 

 ing up to 25 feet in thickness. Pebbly sandstones are 

 of frequent occurrence 10,000 (?) 



3. A Shale Group, consisting of a mingled set of sandstones, 

 black and grey shales and mudstones, with thin seams of 

 coal and ironstone. The shales contain several species 

 of Glossopteris (G. browniana, G. indica, etc.j, while 

 Vertehraria (!) and Schizoneura (gonclwanensis!) occur. 4000 (?) 



2. A Lower Sandstone Group resembling the Upper Sand- 

 stones closely, but with perhaps fewer pebble-beds. 

 The Lower Sandstones are also more conspicuously 

 current-bedded, and, unlike the upper group, have not 

 been proved to contain fossil wood 6000 



1. A group of coarse boulder-beds, black carbonaceous 

 shales, and conglomerates, forming a small isolated 

 block completely surrounded by gneiss. This group 

 is only found near Nacbipere, and its boundaries seem 

 to be faults, but it is presumably the basal member of 

 the Karroo sequence. Bright streaks of coal occur 

 with the carbonaceous shale at one point 450 



The general structure of the Shire district is fairly simple. In 

 the north the Karroo beds are faulted against the eastern gneiss 

 and dip on the whole westwards, with the result that the newest 

 beds arc found in this direction. Farther south a west-south- 

 westerly fault throws back eastwards the Upper Sandstone Group, 

 and this now forms a synclinal basin with the lavas in its centre. 



The Upper Sandstones are overlapped by the recent alluvium of 

 the Shire River, and farther south are faulted against the gneiss 

 of the Port Herald Hills. Patches of Lower Sandstones, however, 

 crop out along this fault-line. A small outlier of possibly basal 

 Karroo is found at Nachipere in the Port Herald Hills, apparently 

 isolated by faults. 



The following sections illustrating the structure of the Shire 

 area will now be described : — 



(A) Sumbu Section, comprising Groups 2. 3, & 4. 



(B) Murukunyama, comprising Groups 3, 4, & 5. (Section 3, fig. 9, p. 218.) 



