﻿196 MESSRS. A. R. ANDREW AND T. E. G. BAILEY [May I9IO, 



narrow dyke-like masses of norite are found in the Shire Highlands, 

 and contain a small quantity of nickeliferous pyrrhotite. Farther 

 north an intrusion of gabbro, showing no foliation, was noticed in 

 the Lower Henga Valley, east of the Nyika Plateau. 



Class (iii). — Coarse-grained, often pegmatitic granites are found 

 occasionally, as near Dowa and Deep Bay. The felspars of the 

 Dowa mass are chiefly microperthite and microcline. 



Age of the plutonic masses. — The age of the plutonic intru- 

 sions will probably never be definitely settled. It is true that no 

 instance is known of a typically plutonic rock having been intruded 

 into Karroo or later beds, but the poor development of unaltered sedi- 

 ments, other than the most recent, might account for this. There 

 can be no doubt, however, that the highly foliated granites of the 

 northern area are of pre-Karroo age. West of the Nyika Plateau, 

 foliated granites form the floor of the Karroo Series, and boulders 

 of the granite are found in the basal conglomerates. 



Tn the case of the foliated granites the question arises, as to 

 whether the foliation practically synchronized with the intrusion 

 of the granite, or originated after its solidification. This is satis- 

 factorily settled for the Dzalanyama intrusion by the discovery of a 

 pegmatite vein, which has been displaced by a series of small tear- 

 faults and foliated across its length in a direction parallel with the 

 foliation of the granite. It is obvious, in this case at least, that 

 the foliation post-dated the solidification of the granite. 



That the foliated intrusions of Class (i) are of pre-Karroo age 

 may be taken as certain. It would, however, be a matter of pure 

 speculation to assign any limits to the age of the plutonic rocks of 

 Class (ii). The rocks of this class are typically alkaline in cha- 

 racter, and show a certain magmatic resemblance to the Tertiary 

 and recent lavas of the Great Rift Yalley system. This resemblance, 

 however, loses most of its significance when one remembers that in 

 the Transvaal nepheline-syenites and other alkaline intrusions form 

 part of the ' Red Granite,' which is considered to be of pre-Karroo 

 age. Moreover, the foliated granites and syenites of Northern 

 Nyasaland belong to an alkaline set of intrusions. We have already 

 referred to the association, in the Shire Highlands, of nickeliferous 

 pyrrhotite with certain intrusions of norite. A similar association 

 occurs in the norites of the Transvaal, which belong to the ' Red 

 Granite ' suite of intrusions. 



On the whole, it appears probable that the plutonic rocks grouped 

 under Classes (ii) and (iii), while later than those of Class (i), are 

 themselves of pre-Karroo age. 



Quartz and pegmatite-veins. — A description of the crystal- 

 line rocks of Nyasaland would not be complete without some 

 mention being made of the numerous veins of quartz and pegmatite 

 which traverse the older rocks. There appears to be no clear line of 

 separation between quartz and pegmatite-veins, for the one shades 



