182 CLEAVAGE IN TILLITE 



angles to their dip. This causes the tillite to weather into 

 lenticular slabs, which are very characteristic of the out- 

 crops in the southern parts of the Colony. This slab or 

 "tombstone" structure, as the late Prof. Green called 

 it, is shown on Plate XIII. In the Karroo outside Kar- 

 roo Poort, where the Dwyka tillite has been affected 

 by the pressures that produced the east and west folds 

 (Zwartberg folds) and also by those which gave rise to 

 the north and south folds (Cederberg folds), the rock 

 has the rough cleavage developed in two directions and 

 weathers out into pillars, usually tapering upwards. 

 The development of the slab-structure becomes weaker 

 as the formation is followed northwards into the region 

 where folding did not take place. 



A curious feature in both the northern and southern 

 boulder-beds, but more highly developed in the latter, is 

 the regular and close jointing of the inclusions. A pebble 

 only a few inches long may be traversed by a dozen or 

 more joints parallel to one another, and quite independent 

 of the original divisional planes, such as those of bedding 

 or foliation, in the pebble. In the north and north-west 

 where the boulder-bed lies nearly horizontally, the joints 

 are also horizontal, but occasionally vertical ones can be 

 found. 1 In the south the joints, which are parallel in 

 all the pebbles at any one spot, lie more or less parallel 

 with the strike, but not with the bedding planes in the 

 boulder-bed. Occasionally one or more of the sections 

 into which the pebbles are divided have been shifted re- 

 latively to the others ; the matrix of the rock shows no 

 sign of the continuation of the joints through it. 



1 See Plate V., T. S. A. P. S., vol. xiv., p. 402, 1903. 



