PRE-CAPE ROCKS OF SOUTH AND WEST OF COLONY 43 



Grobbelaar's River and Matje's Kiver. The felspar 

 occurs in fragments of such size and form that in 

 places the rock has the appearance of a porphyritic 

 granite. When examined under the microscope in thin 

 sections the quartz and felspar are seen to be broken 

 crystals, although the crystalline form of the quartz is 

 occasionally, seen. The felspar is mostly microcline, 

 but albite is frequently, and orthoclase occasionally, 

 met with. These minerals are enclosed in a ground 

 mass chiefly composed of small grains of quartz and 

 minute flakes of sericite, a pale micaceous mineral ; small 

 flakes of brown mica are sometimes found taking the 

 place of the sericite. The mica forms a thin casing round 

 the large grains of quartz and felspar, and the two latter 

 minerals are often seen almost in contact with a very 

 thin film of sericite between them. The sericite occurs 

 in this rock in the same manner as in many gneisses 

 and conglomerates that have been subjected to great 

 pressures in the earth's crust. In some localities the 

 rock shows a distinct schistosity, and in thin sections 

 the large quartz fragments are seen to be elongated in 

 the plane of schistosity, and have patches of interlock- 

 ing grains of quartz at their two ends, as if the material 

 had been removed from the sides of the fragments and 

 deposited at the ends. The minute sericite flakes lie 

 in one direction, along the planes of schistosity. The 

 quartz-felspar rock of the Cango appears to have been 

 a sedimentary rock composed chiefly of fragments of 

 quartz and felspar, in which the micaceous minerals 

 have been developed by pressure ; it seems therefore 

 to be an indurated arkose. This view is supported by 



