﻿Vol. 66.'] GEOLOGICAL STRUCTURE OF SOUTHERN RHODESIA. 363 



in the Essexvale area already mentioned. Epidote is also locally 

 very abundant, and zoisite is sometimes seen. 



It is, of course, these mixed rocks and the gneissic edges of the 

 granites themselves that correspond with the original ' Lower 

 Laurentian ' or * Fundamental Gneiss ' of Canada and other regions. 

 It is important, therefore, to notice the clearness of the evidence 

 regarding their relations to the schistose rocks. After a little 

 practice, it becomes easy to predict the nearness of the schists when 

 travelling over grauite and the approach of granite when travelling 

 across the schists. In the case of the common epidiorites, for 

 example, one notices that they usually become more foliated in 

 character and that epidote often makes its appearance. Augite 

 may be developed from the uralitic hornblende x and sphene from 

 the ilmenite, the rock gradually merging into a coarse hornblendic 

 or pyroxene-granulite. In the latter case enstatite is frequently 

 present, in addition to augite. 2 Biotite and garnet may also appear, 

 while patches and nodular masses of the so-called 'eclogite' may 

 occur, as at Mbanji in the Wankie district. Where injection- 

 processes are especially prominent, little veins of quartz and micro- 

 cline (or other acid felspar) may be detected along the foliation- 

 planes, as well as actual dykes of more or less normal granite, or of 

 an aplitic or pegmatitic nature. These act as feeders to the pro- 

 cesses of injection, and the absence of movement as an aid to the 

 production of a gneissic or banded aspect in the adjacent granite 

 is proved by the fact that these offshoots from it are rarely crushed 

 or crumpled in any way. As the main granite mass is approached, 

 interlaminar injection may give rise to the production of a typical 

 ' banded gneiss,' the light bands being principally granitic in origin 

 and the darker bands being composed of hornblende, etc. derived 

 from the original epidiorite. Inside what may be considered as the 

 granite boundary, patches and shreds or streaks of the basic material 

 may be abundant. The whole character of the granite itself near 

 by is often affected by the absorption of the material of which these 

 remnants are visible. Thus, at nearly all contacts with hornblendic 

 rocks, the granites are hornblende-bearing and rich in plagioclase. 

 They also often show the rather unexpected feature of orthoclase 

 co-existing with or entirely replacing microcline. All the really 

 normal granites (that is, the interior portions of the great masses) 

 are biotite-bearing and have microcline as the dominant felspar : 

 they never contain hornblende or muscovite. The gneissic ap- 

 pearance of the edges of the granite masses has no connexion with 

 shearing or crushing in the ordinary sense, although they often 

 exhibit the most typical foliated and ' augen ' structures. This is 

 plainly shown by the entire absence of deformation in the minerals 

 of first consolidation (as, for example, hornblende, biotite, and 

 sphene) ; and also by the fact that the ' augen ' of felspar have no 

 tails of sheared material, being obviously either closely allied to true 



i See W. F. Smeeth, Bull. Mysore G-eol. Dept. No. 3, 1905. 

 2 SeeF. D. Adams, Ann. Rep. Geol. Surv. Canada for 1895 (1897) pt. J, 

 p. 75. 



2c 2 



