﻿Vol. 66.~] OX THE GEOLOGY OF XYASALAXD. 221 



Dykes in the gneiss of the Lower Shire District. — 

 Though dykes are rarely found either in the Lower or in the Upper 

 ♦Sandstones, they occur in great numbers in some of the neighbouring 

 gneissic areas. Thus, between the Mwanza River and Chikwawa, 

 dykes of basalt are exceedingly numerous. In the Chikwawa Hills 

 some of these intrusions are composite, with broad fine-grained 

 margins or fine-grained interior. The Chikwawa dykes are probably 

 connected with the emission of the Karroo lavas. The absence of 

 tuffs among these lava-flows and the uniformity in character of the 

 latter over wide areas suggest that the lavas are the outcome of 

 fissure-eruptions. 



It may be remarked that un foliated dykes are comparatively 

 rarely found in Northern Nyasaland, even in the gneiss ; and, with 

 the exception of the Blount "Waller area, no dykes are known in 

 the Karroo. 



Comparison of the Southern and Northern Karroo 

 of Nyasaland. — In the north it was found possible to separate 

 roughly the Karroo into a lower sandstone division, a middle shale 

 division, and an upper sandstone and calcareous division. In the 

 south a somewhat similar sequence is observed, but the upper and 

 lower sandstones, in common with the intervening shale group, are 

 vastly thicker than their northern analogues. Again, the limestones 

 and calcareous mudstones, so characteristic of the upper portion of 

 the Karroo of Mpata and Nkana, appear to be unrepresented in 

 the south ; yet a few thin beds of oolitic and argillaceous limestone 

 do occur in the Upper Sandstone Group near Nkombedzi villages. 

 On the other hand, the thick lava group of the southern succession 

 is missing in the north. It is possible that lavas once filled the gap 

 which now exists between the Karroo of Mpata and the overlying 

 recent deposits, but the general scarcity of dykes or sills in the 

 north, coupled with the absence of derived pebbles in the overlying 

 recent conglomerates, rather indicates that vulcanicity in Karroo 

 times was restricted to the south. 



It will be seen, then, that no very close comparison can be made 

 between the Karroo of Northern and that of Southern Nyasaland. 

 In Rhodesia, Mr. Molyneux l has described a series of basal con- 

 glomerates followed directly by a coal-bearing group (Lower Mato- 

 bola Beds), which is overlain by a group with limestones (Upper 

 Matobola Beds), grits (Escarpment Grits), sandstones (Forest Sand- 

 stones), and, finally, by the Tuli or Batoka lavas. The Matobola 

 Beds contain freshwater shells belonging to the genus Palceomutela 

 and fish-remains : they are therefore similar, in fossil contents, to 

 the calcareous group of Nkana and Mpata in Nyasaland. 



The apparent thickness of the Karroo in Rhodesia seems to be 

 commensurable with that of Northern Nyasaland ; moreover, the 

 position of the coal-bearing group, lying, as it does, directly upon 

 the basal conglomerates without the intervention of a thick series 



1 Quart. Journ. Geol. Soc. vol. lxv (1909) p. 421. 



