﻿200 



THE PSEUDOTACHYLYTE OF PAEIJS. 



[vol. lxxiu 



unevenly distributed, and is especially intense in the neighbourhood 

 of cracks. The larger crystals often pass externally into micropeg- 

 matite. The grey bands contain much oligoclase with practically 

 straight extinction, also a variable but generally large proportion 

 of biotite which often exceeds 50 per cent. I have not observed 

 any hornblende in the few slices that I have cut ; but at the weir, 

 2 miles above the township, bands of a bright-green amphibolite 

 occur in the granite. 



The special interest of the district attaches, not to the granite- 

 gneiss, but to a remarkable system of veins of apparent tachylyte 

 which intersect the granitic rocks everywhere throughout the area 

 north of Vredefort. Of the southern portion of the area, where 

 exposures of the granite seem to be much less numerous, I am 

 unable to speak. The best exposures are seen in the bed and banks 

 of the Vaal, where the scour of the running water cleans and 

 smooths the rock-surfaces ; the veins then show up jet-black and 

 exhibit a highly polished surface, thus affording a strong contrast to 

 the rougher grey surface of the granite. Elsewhere they weather 

 grey or greenish, and are then difficult to distinguish from the 

 granite. On each of my visits I took a large number of photo- 

 graphs of these veins, but many of the exposures were so difficult 

 to ' catch ' (on account of their awkward situations, irregular sur- 

 faces, slight colour-contrasts, and so on) that I found it desirable 

 to supplement my photographs hy scale-drawings, some of which 

 are reproduced herewith. 



The highly irregular, branching intrusion of PL XVI was ex- 

 posed in a deep cutting made for one of the piers of the new 

 bridge ; the rock-face shown is about 7 feet high, which will give 

 a measure of the dimensions of the intrusion. This cutting has 

 since been tilled up. The photograph reproduced in PI. XVII, 

 fig. 1 was obtained in another excavation near the first ; it shows 

 a vein turning from a vertical to a nearly horizontal plane and 

 thinning out. The width of the vein is 6 inches at the centre of 

 the photograph. The detail of the blind end of this vein is shown 

 in text-fig. 8 (p. 203). PI. XVII, fig. 2 sIioavs a large block which 

 has been thrown out by blasting operations at the weir, above the- 

 town ; the entire block is only a portion of a wide dyke of pseudo- 

 tachylyte. The floaters of granite which are embedded in the dark 

 base are so numerous and so perfectly rounded that the rock 

 resembles in appearance a sedimentary conglomerate. This is a 

 most remarkable specimen, and, despite its weight, which must be 

 about a ton, one Avould like to see it removed entire to a museiun. 



The characteristic features of the intrusions, as seen in the field, 

 are summarized in the following paragraphs : — 



The veins are utterly irregular in form, direction, and thickness (text- 

 figures 6, 7, 10-13 ; PI. XVI). 



They have every inclination from vertical (PI. XVI) to horizontal (PI. XVII, 

 fig. 1), and they strike towards all points of the compass. 



They change their direction again and again, and often follow sinuous 



