708 MR. E. H. RASTALL ON THE GEOLOGY OP [Nov. I9II, 



V. The Petrography op the Malmesbury Rocks op the 

 Worcester District. 



As we have already seen, the Malmesbury rocks of the Worcester 

 district can be divided on general grounds into two series, which 

 differ somewhat markedly in lithological character. The lower 

 division is best seen in the fine road-section along the right bank 

 of the Hex River, north of De Wet Station, while the upper series 

 is well displayed in the numerous kopjes which form the rolling 

 ground extending for some miles due north of Worcester. The 

 massive grits at the base of the whole are also now well exposed, 

 in the excavations for the new waterworks at the head of the 

 Hartebeest River. A short distance north of Worcester bands of 

 limestone and masses of intrusive igneous rock occur in the upper 

 series; while the igneous rocks of Brewels Kloof form an excep- 

 tional type, and need separate treatment. 



(1) The Sedimentary Rocks. 



(a) Lower division : the gritty series. — The rocks forming 

 this series are on the whole very uniform in character, the chief 

 variations being in the matter of colour, which is obviously of 

 subsidiary importance. They may be collectively described as 

 grey, greeu, or purple grits, usually massive and rather fine in 

 texture [9463]. 1 When weathered, they often take on a brownish 

 colour; and here and there comparatively thin bands of white or 

 very pale grey quartzite are met with, as for example near the 

 farmhouse Zeekoe Gat. Passing southwards, these grits and 

 quartzites gradually give place to a great thickness of flags and 

 gritty slates, usually dark grey. These are very different from 

 the silvery slates and phyllit.es of the upper division, which begin 

 about half a mile south of De Wet Station. 



A typical specimen of one of the grits from a short distance 

 north of De Wet [9459], when examined microscopically, is seen to 

 consist for the most part of angular and subangular grains of quartz, 

 with abundant small flakes of a yellowish mica and occasional grains 

 of tourmaline, zircon, epidote, etc., which are clearly of detrital 

 origin. In some specimens, but not universally, grains of felspar 

 occur in small quantity. The grains are somewhat sparsely dis- 

 tributed in the cementing material, which is relatively abundant. 

 The cement is very fine in texture, so that its constituents are 

 somewhat difficult to determine. It appears to be chiefly composed 

 of quartz and mica in minute grains and flakes, but a good deal of 

 some irresolvable argillaceous material also seems to be present, 



It is noticeable that all the larger quartz-grains show a certain 

 amount of corrosion at the edges, as if some kind of reaction had 

 taken place between the quartz-grains and the constituents of the 

 cement. This undoubtedly indicates incipient metamorphism, but 

 whether this is thermal or dynamic in character, it is impossible to 

 say with certainty. 



1 The numerals in brackets refer to slides in the Sedgwick Museum col- 

 lection, Cambridge. 



