p.irt 1] AND ASSOCIATED ROCKS OF MOZAMBIQUE. SI 



resist weathering as well as the rock through which they have 

 intruded, for they show no signs of alteration beyond an iron- 

 stained selvage 2 mm. thick or less, and they make no feature 

 on the ground, either positive or negative. 



Hand-specimens have a greenish-grey colour, and are of uniform 

 saccharoidal grain (about 1mm.). The average specific gravity 

 of three specimens is 3*21. Under the microscope the rocks are 

 found to be all very similar, and No. 33 may be taken as a type 

 representative of the whole series (PI. X, fig. 5). 



Diopside, colourless or nearly so in thin section, is by far the most abundant 

 mineral. It builds a roughly equidimensional mosaic of sub-rectangular or 

 rounded individuals. Some of the diopsides are seen to have yellowish-green 

 slightly-pleochroic inclusions. Under crossed nicols these patches give 

 aggregate polarization, and may be referable to serpentine or chlorite. They 

 probably represent the alteration-products of amphibole : for, in places, simi- 

 larly shaped inclusions occur consisting of blue or greenish-blue amphibole, and 

 passing into altered material at the edges. The amphibole has a maximum 

 measured extinction of about 10°, and its pleochroic scheme is as follows : — 



X = a yellowish-green. 

 Y = b greenish-blue. 

 Z = c indigo to violet. 



In places a few rounded crystals of enstatite occur, distinguishable by their 

 straight extinction, lower refractive index, and much lower birefringence. 

 Neither olivine nor felspar has been detected. 



In the Mluli Yalley east of the Mwipi Range, are dykes of 

 felspathic pyroxenite which may be conveniently described here, 

 although their age is not known beyond the fact that they are of 

 post-gneiss age. Colourless diopside is again the chief mineral 

 present, and in places it constitutes practically the whole of the 

 rock, which is of a uniform grey colour. 



In thin section (No. 184) it is seen that a few granules and plates of calcic 

 labradorite or bytownite are present. Swarms of minute gTanules of pyroxene 

 occur in the plagioclase plates, which are thus reduced to a sponge-like 

 skeleton. The only other minerals present are a few shreds of biotite (also 

 carrying inclusions of granular pyroxene), occasional enstatites as in tbp 

 Xrassi pyroxenites, and occasionally a rounded apatite. 



Pyroxenite-dykes occur in Ceylon, and have been described by 

 A. K. Coomaraswamy. 1 Although many of them differ markedly 

 from the Xrassi type by the possession of phlogopite and several 

 other minerals, others seem to be of identical tvpe, for it is stated 

 that 



' a nearly colourless monoclinic pyroxene [is] the chief or only constituent of 

 many of the most conspicuous varieties.' 



A more remarkable point of resemblance lies in the position in the 

 sequence. In Mozambique the pyroxenites follow the succession 



(1) gneisses, (2) granuJitic granites, and (3) massive pegmatites; 

 while in Ceylon they follow the similar succession (1) gneisses, 



(2) granulites (charnockite series), and (3) massive pegmatites. 



1 Geol. Mag. dec. 5, vol. ii (1905) p. 363. 



Q. J. G. S. No. 293 o 



