462 C. B. Honvood ^ A. Wade— The Old Granites of Africa. 



the feldspar through decomposition. Broken needles of rutilc in the 

 quartz grains and contorted mica folia, as well as the bent and broken 

 feldspars, combined with undulose extinction of tlie quartz and feld- 

 spars, indicate considerable cataclastic action. This probablyexplains, as 

 Molengraaif observes, the cross-hatching structure of the microcline ". 



Dr. Yoit,^ speaking of the Old Granite of German South- West 

 Africa, says that the frequent occurrence sometimes in veins and 

 sometimes in great masses of pegmatite is characteristic. He states 

 that mica occurs as large as a man's hand, and a red felspar 

 (microcline) ot gigantic size, like graphic granite, with quartz 

 inclusions ; that the mica is chiefly muscovite, accompanied sometimes 

 by biotite in small flakes, sometimes, too, small flakes of a green mica 

 occur in the graphic granite, in which the blow-pipe aiSords evidence 

 of chromium ; that tourmaline is a very common constituent of the 

 granite, and occurs in both large and small individuals up to an arm's 

 length and thickness ; that clusters of small apatite crystals are often 

 to be found in the red felspar ; and that topazes and beryls are found 

 as long as one's hand, the latter being of a completely opaque and 

 dirty variety, the clear, bluish-green, glassy variety of aquamarine 

 being seldom found. He also describes the gneiss-granite district 

 as unusually rich in secondary minerals, the felspars near Ganab 

 being frequently changed to zeolites. 



Mr. A. W. Eogers,- Director of the Geological Survey of the Cape 

 Colony, in describing the granite massif which stretches from St. Helena 

 Eay to Klein Dassen Berg, a distance of about 70 miles, says that 

 many varieties of granitic rock occur, the most abundant being, 

 perhaps, a two-mica granite, with orthoclase as the chief felspar. 

 He states that every gradation between a normal granite and a gneiss 

 can be found, and that the massive granite is seen in the interior of 

 the area and the foliated rock near the periphery, but that this rule 

 has many exceptions. He says that large and small veins or dyke- 

 like bodies of microgranite and quartz-porphyry, with a microgranite 

 base, are found towards the edge of the area in many places ; and 

 tourmaline is often present in the rock near Darling. Also that in the 

 hills to the south and west of Darling there are some remarkable rocks 

 associated with the granite and gneiss ; colourless augite, plagioclase, 

 and sphene are added to the usual constituents of the granite, and mica 

 is practically absent; the structure is that known as granulitic, the 

 various minerals occui'ring in grains of a more uniform size than is the 

 case with ordinary granite. These rocks often show a parallel structure, 

 but have not the foliated or schistose planes seen in the gneiss. 

 He adds that the nature and origin of the granulites of Darling are 

 as yet unexplained, as is also their exact relationship to the surrounding 

 granite and gneiss. 



With regard to the Malmesbury granite boss, he says that it is mainly 

 a rather coarse biotite granite with porphyritic orthoclase, but fine, 

 grained granite, composed of the same minerals, and coarse pegmatites 

 are not infrequent, and that there seems to be no gneiss. 



1 " A Contribution to the Geology of German South-West Africa," by Dr. F. W. 

 Voit: Trans. Geol. Soc, vol. vii, ^t. ii, pp. 84, 85. 



- The Geology of Cape Colotiy, by A. W. Eogers, pp. 38-42. 



