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



beds themselves, owing chiefly to their inaccessible and unhealthy 

 situations, are still very imperfectly known. The main interest of 

 the Rhodesian region, from a geological point of view, consists in 

 the fact that it is the first part to be examined in any detail of 

 what may be termed the Laurentian area of Africa, which 

 is probably as large and as interesting as that of North America 

 itself. 



During the past eight years I have spent practically the whole 

 of the time that I could snatch from official duties in field-work 

 among the Rhodesian rocks, supplemented by examination of them 

 under the microscope. A number of previous papers have dealt with 

 certain of the observations made, but the present communication is 

 the first that embodies the results of the actual mapping of any 

 considerable portion of the country. It will, of course, be realized 

 that a region possessing all the complexity of the Scottish Highlands, 

 and many times as extensive, will need far more detailed study 

 before our knowledge of its problems can be considered at all 

 complete. At the same time, certain generalizations can be made, 

 some of them bearing on questions of fundamental importance in 

 Archaean geology. A brief account is also given here of the sedi- 

 mentary rocks of the area and of the igneous rocks which form so 

 important a feature of the country. 



II. The Metamorphic Rocks. 



The oldest rocks of Rhodesia comprise almost every conceivable 

 lithological type, and exhibit nearly every possible degree of alter- 

 ation. It would be fruitless to attempt their minute subdivision or 

 even to endeavour to define the areas occupied by the main types. 

 It is only possible at present to point out certain facts which have 

 gradually been ascertained by study in the field and under the 

 microscope, and afford an obvious basis for working out the 

 structural features of the whole country. 



(1) The Hornblendic Rocks. 



First of all, attention may be drawn to the great development of 

 hornblendic rocks. Mica-schists, of which we hear so much else- 

 where, are almost unknown ; indeed, I have never been able to- 

 collect as much as a single specimen resembling those so well deve- 

 loped, for example, in the neighbourhood of Salcombe in Devon,, 

 which I examined shortly before leaving England. One finds, 

 moreover, few of the intensely sheared rocks of such a district, 

 except very locally — a few yards, for instance, on each side of a 

 gold-reef, where a well-foliated chlorite-schist may be produced from 

 a massive epidiorite. 



It is the massive epidiorites and amphibolites, like those upon 

 which Bulawayo is built, that constitute one of the most striking 



