﻿362 MR. FREDERIC P. MENNELL ON THE [Aug. I9IO,. 



Til. The Granite Masses, and their Relations with 

 the Met amorphic Rocks. 1 



As will be gathered from a glance at the map accompanying this 

 paper (PI. XXVIII), the granite masses occupy a much larger area 

 than all the metamorphic rocks combined. They are, moreover, 

 equally prominent on the north and on the east of the area mapped, 

 indeed throughout nearly the whole of tropical Africa. It is no 

 matter for surprise, therefore, that Mr. J. A. Chalmers & Dr. F. H. 

 Hatch originally regarded the schists as representing altered in- 

 trusions into the granite. 2 The unfamiliar aspect of the contact- 

 phenomena in an area of crystalline schists also no doubt contributed 

 to this erroneous conclusion. A detailed examination of any 

 contact-zone will, nevertheless, yield good evidence of the later 

 origin of the granite. 3 ' Lit-par-lit ' or interlaminar injection, 

 is the characteristic feature of contacts, together with the pro- 

 duction of 'mixed rocks' due to more or less complete absorption 

 of schistose materials into the granite masses. It is frequently 

 impossible to draw any good line of demarcation between granite 

 and schists, 4 and in detailed mapping it would be necessary to 

 colour separately certain areas as contact-zones. In the present 

 instance, owing to the small scale of the map and the imperfection 

 of our present knowledge, it has not been deemed advisable to 

 attempt this, and a more or less conventional line has been drawn, 

 indicating as far as possible predominance of granite on one side 

 and of schists on the other. Some important areas of mixed rocks 

 are, however, indicated on the map. 



The difficulties of mapping are much increased by the great 

 variability of the distribution of these rocks, even round a single 

 plutonic mass. For instance, the northern contact of the small 

 Heany mass is fairly sharp, and one passes abruptly from pure 

 granite to pure schist. Its southern boundary is, however, 

 extremely indefinite, and there is an area of mixed rocks in the 

 Essexvale district more than equal in extent to the whole of the 

 normal granite. The ' mixed rocks ' in these cases comprise types 

 that would often be classed as aplite, felsite, microgranite, gneiss, 

 quartz-diorite, diorite, amphibolite, hornblende-gran ulite, pyroxene- 

 granulite, eclogite, etc., but as few of these can have really crystal- 

 lized freely from true fusion, the writer prefers to group them 

 all together as granulites. They are characterized, as a rule, by 

 a rather fine, evenly granular structure ; also by the presence of 

 quartz, and especially of microcline, in even the most basic types. 

 It is these rocks that are the home of minerals like garnet, 

 cordierite, fibrolite, kyanite, staurolite, corundum, spinel, etc. 

 Tourmaline and topaz occur in certain of them, as, for example, 



1 Compare Addresses etc., Brit. & S.A. Assoc. Adv. Sci. 1905, vol. ii, 

 pp. 23-37. 



* Geol. Mag. dec. iv, vol. ii (1895) p. 197. 



3 See 1st Ann. Rep. Rhod. Mus. for 1902 (1903) p. 10 ; ' Geology of Southern 

 Rhodesia' 1904 (Spec. Rep. No. 2, Rhod. Mus.) pp. 10-11 ; and Geol. Mag. 

 dee. v, vol. iii (1906) p. 260. 



* B. K. Emerson & J. H. Perry, Bull. U.S. Geol. Surv. No. 311 (1907) p. 46. 



