400 Transactions of the South African Philosophical Society. 



even less pressure than that on the En on Conglomerate, the agents, 

 heat and solution, being equally absent. The split Dwyka pebbles 

 occur in every locality where that conglomerate exists in the south, 

 and where the rock was deposited under water, and the same 

 phenomenon occurs in the north, where the rock is a boulder-clay 

 formed on land. The conditions under which the pebbles have been 

 cleaved is best studied, however, in the north of the Colony, where 

 the shearing and cleavage of the matrix in which they are imbedded 

 does not come in and confuse the results. The cleaved boulders in 

 the Dwyka Conglomerate of Prieska occur in a moderate hard 

 boulder-clay. The rocks that exhibit the cleavage are of all varieties, 

 slate, quartzite, jasper, granite and amygdaloidal melaphyre. The 

 cleavages are nearly always horizontal and are sometimes ex- 

 ceedingly numerous, cutting the rock into numberless thin laminae, 

 at other times only a few joints pass through the pebble. The 

 cleavages cut through all minerals hard or soft equally, in a sharp 

 cut, and are quite independent of the bedding of the sedimentary 

 rocks (see fig. 2, pi. v.). 



In some cases cleavage only is present, in others the laminae are 

 shifted in position. The cleavages do not pass into the matrix, which 

 has all the appearance of being quite undisturbed. From what we 

 have seen occurred with regard to the pencil of slate in the lead, it 

 seems probable that this appearance of homogeneity in the Dwyka 

 matrix is misleading. In reality there are probably faint divisions 

 of the niaterial corresponding to the cooled surfaces in the lead, and 

 each one of these layers acted by itself and endeavoured to carry its 

 included piece of boulder along whatever way the particular layer 

 was spreading. If the flow was small, the rock was simply split ; 

 but if it was at all great, the friction between adjacent laminae 

 was overcome and they were shifted relatively to one another. If 

 there had not been this parting, and the clay had really been 

 homogeneous, it would have flowed round the boulders without 

 disturbing the including fragment, in the same manner that the 

 lead flowed round the pencil in the zone of greatest bulging. 



We have seen, however, that in the case of the lead, that a pressure 

 of 6*72 tons to the square inch was insufficient to cleave the pencil, 

 it simply fractured it. The pressure on the Dwyka boulders was 

 nothing like 6-72 tons. The ice on top could not have been 1,600 

 feet thick, for a greater weight could not be carried by the foot 

 without melting ; and 500 feet would be a liberal allowance for the 

 thickness of glacial conglomerate at Prieska. All this would only 

 produce a pressure of half a ton per square inch at the foot, quite 

 insufficient to break the pebbles in the manner described. Here 



