﻿364 ME. FEEDEEIC P. MENNELL ON THE [A-Ug. I9IO, 



phenocrysts, or being of the nature of injected knots like those 

 often seen among the schists near contacts. The granular quartz 

 and felspar, which sometimes form a kind of ground-mass, show no 

 strain-shadows or distortion of the twin-lamellee. These more 

 finely granular portions often merge into micropegmatite, an un- 

 mistakable indication of the igneous origin of the whole. Patches 

 of micropegmatite (quartz 4- orthoclase or oligoclase) are frequently 

 found included in the plates of microcline (Matopos, Jahonda, etc.) : 

 a fact which has an interesting bearing on theoretical problems, and 

 conclusively proves that a ' eutectic ' was not the final residuum of 

 crystallization in these rocks, where the microcline generally shows 

 a tendency to crystallize later than the quartz. 



It may be remarked that the granite masses as a whole have a 

 much less foliated character than is apparently the case in the 

 Laurentian areas of Canada. There is seldom the slightest trace of 

 a gneissic structure away from the margins of a mass. The only 

 exception to this statement is where there has evidently been a 

 synclinal fold in the originally overlying schists, now entirely 

 removed by denudation, as, for instance, in the Mapane Flats, south 

 of the Matopo Hills, where an east-and-west belt of mixed rocks 

 occurs isolated on both sides by many miles of normal granite. In 

 these gneissic rocks biotite is very abundant, and orthoclase replaces 

 microcline ; while dark bands of hornblende-pyroxene-granulite also 

 occur. 



It may be remarked that the rare mineral orthite, or allanite, 

 often associated with epidote, occurs in most slides made from 

 specimens of the Matopo granite and also in other masses. 1 It 

 usually forms patchily coloured yellowish grains or crystals about 

 a millimetre long, very feebly pleochroic, and having a double 

 refraction not exceeding that of quartz. Sometimes, indeed, it is 

 quite isotropic. Both the orthite and the epidote which sometimes 

 surrounds it are frequently idiomorphio towards biotite (many 

 Matopo localities) aud also towards hornblende (near Zimbabwe 

 Euins), and have every appearance of being primary constituents, 

 although one cannot avoid a suspicion that they may arise from 

 some kind of leaching process during the cooling of the rocks. 

 Orthite occurs in a granophyric (microgranitic) intrusion among 

 the schists at the Morven Mine near the Bembezi Eiver, the only 

 instance with which I am acquainted outside the great granite 

 masses. In a granite from Bumbuzi, near the Deka Eiver, west of 

 Wankies, garnet is abundant as well as orthite, an association 

 which I have also seen in more than one rock from North-Eastern 

 Ehodesia. Apart from the Matopos, the orthite seems usually 

 confined to the marginal modifications of the granite masses. 

 Altogether 15 per cent, of my slides contain the mineral, which 

 has not hitherto been recorded from the granites of any other part 

 of South Africa, although I have noticed it in a slice from a rock 

 near the Sh&shi Eiver in the Tati district of Bechuan aland. 



1 See Geol. Mag. dec. iv, vol. x (1903) p. 347, & Eep. S.A. Assoc. Adv. Sci. 

 1903 (Oape Town) p. 284, with fig. 



