PRE-CAPE ROCKS OF SOUTH AND WEST OF COLONY 37 



rock, and the hornblende often encloses the felspar 

 crystals, so as to give the rock a partly ophitic struc- 

 ture ; some large crystals of mica (biotite) behave in 

 the same way. Most of the felspar belongs to the 

 oligoclase series of the plagioclases, but there are 

 patches of a very much altered felspar, strongly con- 

 trasted to the clear crystals of plagioclase, which are 

 very probably orthoclase. Quartz is present in consider- 

 able quantity, filling up the spaces between the other 

 minerals. The rock is little altered as a whole, but some 

 of the mica is replaced by chlorite, and some epidote, 

 derived from the alteration of other constituents, is 

 present. Another variety of diorite in this neigh- 

 bourhood contains the same minerals as the one just 

 described, but monoclinic pyroxene, with the character- 

 istic diallage structure, is present in considerable quan- 

 tity, forming in thin sections ophitic plates enclosing 

 felspar. The pyroxene sometimes forms complicated 

 intergrowths with the hornblende and also occurs in the 

 centre of large hornblende crystals ; in such cases one 

 set of prism cleavages is common to both minerals. 



In the gneiss of Klein Dassen Berg there is a dioritic 

 dyke intruded parallel with the foliation planes of the 

 gneiss. The rock of this dyke is rather different from 

 the Klein Paarde Berg rock, in that the constituent 

 minerals, plagioclase, hornblende and quartz, form 

 nearly equal- sized grains, and none of them have any 

 proper crystal faces ; the structure is typically grahulitic. 

 At Yzer Fontein Point is a large mass of hornblendic 

 rock, coarsely crystalline, with a banded structure ; some 

 thick layers are formed entirely of green hornblende, 



