part 2] i-eoi.im.v or the m lhble delta. l-"> 



Special interest attaches to the occurrence regarded by Dr. Hatch 

 as a granite-boulder embedded in the marble, the concentric rings 

 of minerals surrounding it being lie Id to have resulted from the 

 reaction between the solid inclusion and 1he enclosing dolomite 

 under the thermal action of the underlying intrusive gneiss and 

 granite of the region. 



From the description given by him, the occurrence perforce had 

 to be correlated with that found in the uppermost quarry on 

 The Glen — the only quarry that was being worked in 1909-10, — 

 the same association of rock-types being present, although the 

 soda-granite was undoubtedly intrusive there. It now became 

 apparent that the so-called ' boulder ' must actually have been the 

 tip of a granitic sill intruded almost parallel to a bedding-plane in 

 the marble, by which the latter had been altered in the manner 

 characteristic of igneous contacts. 



A sketch of the exposure as it appeared in August 1916 is given 

 in fig. 3 (p. 126), regarding which it must be borne in mind that the 

 rock-face is not quite Hat, and that the sill is dipping away from 

 the observer as well as to the right. The downward continuation 

 is hidden by the debris upon the floor of the quarry ; but, on pro- 

 ceeding down the steep overgrown bank to the river about 130 feet 

 below, I found three small granitic sills from 1 to 5 feet broad 

 close together injected along bedding-planes at about the horizon 

 of the stratum in the quarry above. 



Although the largest of them differs entirely in lithological 

 characters from the granite in the quarry, there is little doubt in 

 my mind that the intrusions are directly connected, and that 

 slight assimilation at the active edge of the sill and partial isola- 

 tion of the magma during its crystallization account for the 

 abnormal constitution of the latter. The margins in the quarry 

 are fairly sharp but deeply embayed, lobes of the granite have 

 clearly eaten their way into the dolomite, while at a distance of 

 about 30 feet to the left, in the position where it might be 

 anticipated, a continuation of the injection is indicated by a patch 

 on the quarry-wall, lenticular in plan, exhibiting symmetrically 

 arranged zones of contact-minerals just as around the exposed 

 t<»ngue, clearly showing that the granite is only a few inches 

 below the surface here. 



The tongue consists of a pale rock with small deep bluish-black 

 specks of ferromagnesian mineral rather like a riebeckite-granite in 

 the hand-specimen : it is generally fine-grained, somewhat streaky 

 at the top, while in places, particularly along its centre, it is 

 coarser in texture, and composed almost wholly of white orthoclase- 

 Eelspar and rather abundant quartz. 



Sections 3382 & 3111, x cut from the granite, correspond very 

 closely indeed with the description given by Hatch & Rastall of 

 the 'inclusion/ except that the quartz is somewhat more rounded 

 in outline; the cloudy orthoclase has, as they state, a delicate 



1 The numbers refer to the slides in the Geological Survey Office ;it Cape 



Town. 



