Vol. 67.] WORCESTER, ROBERTSON, AND A8HTON (CAPE COLONY). 71 1 



shows a regular gradation of characters, from, an almost uncrushed 

 quartz-porphyry as above described to a highly schistose rock, con- 

 sisting for the most part of finely granular quartz and felspar and 

 sericitic mica, in which the original phenocrysts are represented 

 by much -flattened augen of recrystallized quartz and felspar. 

 The sericitic mica, which is pale brown or colourless, forms very 

 conspicuous streaks curving round the augen. 



There is hardly sufficient evidence available to decide with 

 certainty what the form of these intrusions may be, but it seems safe 

 to conclude that they are for the most part of dyke-like habit. At 

 the marble-quarry there is evidence of a considerable degree of 

 disturbance of the strata and also of strong thermal metamorphism ; 

 in all probability these granite dykes are offshoots of a larger and 

 more deep-seated mass which is not exposed at the surface : the 

 form of the intrusion, as indicated in fig. 1 (p. 705), is not to be 

 taken too literally, as it is merely diagrammatic and to a certain 

 extent hypothetical. 



(3) The Igneous Rocks of Brewels Kloof. 



The exact size and boundaries of this mass are somewhat 

 difficult to determine, owing to the roughness of the ground and the 

 poorness of the exposures in some parts, but it appears to be some- 

 what elliptical in form, and about three-quarters of a mile long by 

 half a mile broad. Where the contact of the igneous and sedi- 

 mentary rocks can be seen, the former appears to be always inclined 

 at a high angle, sometimes nearly vertical, and the outer portions 

 of the mass show a good deal of shearing. For this reason, it is 

 difficult to determine whether there is any difference of texture 

 between the outer and the inner portions of the mass. 



A typical specimen of this rock, when unweathered, is dark blue 

 or grey in colour, heavy and compact, with a spotted appearance, 

 which, as can be seen from microscope-sections, is partly due to 

 porphyritic felspars, and partly to vesicles filled with secondary 

 minerals. AYhen weathered, these secondary minerals offer less 

 resistance than the rest, so that the exposed surfaces frequently 

 show a very conspicuous vesicular pitting. 



Thin slices of this rock [9464-65] reveal abundant phenocrysts 

 of plagioclase-felspar and pseudomorphs after some ferromagne- 

 sian mineral, together with rounded or oval vesicles, in a ground- 

 mass which varies considerably in character owing to different 

 degrees of decomposition. 



In some specimens the felspar-phenocrysts are quite fresh, and 

 show extinction- angles corresponding to andesine and labradorite. 

 In other cases the felspar has become more or less turbid, owing to 

 conversion into an aggregate of minute crystals of mica and zoisite. 

 This gives a false appearance of strong refringency, and causes the 

 crystals to present a curious similarity to andalusite. 



The ferromagnesian mineral is now represented by chloritic 



