360 ORIGIN OF RAXD BANKETS. 



Looking at the subject from another standpoint, it is incon- 

 ceivable that a vast mass of igneous rock — taking it as a molten 

 mass on the old view, or as a solution containing a corrosive acid 

 like silica on the newer view — should flow over and rest upon a 

 mass of ferruginous and calcareous rocks without absorbing 

 them. Even laterally, without the aid of the pressure derived 

 from the weight of the liquid mass, the sediments are profoundly 

 metamorphosed and in part recrystallised : add to the metamorphic 

 action an additional provocative of change such as an increase of 

 pressure when the liquid mass wedged its way between the 

 Pretoria Beds below and the Waterberg Beds abova, and actually 

 lifted up the latter, then it is inconceivable that the Pretoria and 

 Dolomite Beds could withstand being absorbed. 



The Transvaal laccolite differs from the ordinary type of 

 small laccolite in that the vast size of the mass has caused the 

 bottom to be convex and not a straight line, while the arching of 

 the top. judging from the small portions of the cover still re- 

 maining, was not very pronounced. In other words, one has to 

 turn the ordinary diagrams of laccolites of the Henry Mountain 

 type upside down to obtain an idea of the nature of the Trans- 

 vaal laccolite. and this is due to the weighting of the crust by the 

 additional lode, and perhaps by the material from below, on being 

 transferred to a higher horizon, leaving a hollow which caused 

 the sedimentary beds to curve downwards ; certainly the beds of 

 the Transvaal System dip towards the laccolite all round the 

 margin. 



The crowd of sills of diabase that penetrate the sedimentary 

 beds round the margin belong to the laccolite, and can be 

 regarded as a lit- par-lit injection on a large scale. Similar 

 occurrences have been described by Pirsson from the Judith 

 Mountains in Montana, but these are very small in comparison 

 with the Transvaal laccolite. 



Evidences of thrusting on the south of the laccolite, and 

 caused by the laccolite, are very plain ; the Pretoria Dolomite and 

 Black Reef Beds immediately about Pretoria run east and west, 

 the folds being huddled up against the block of granite that has 

 caused an obstruction. To the east, however, where the influence 

 of the granite is not felt, the strike of the beds splays out in a 

 very marked manner. The dykes of diabase follow strictly the 

 strike of the sedimentary beds both in the place where they run 

 east and west, and where they splay out to the south-east ; there- 

 fore the intrusions of the sills was contemporaneous with the 

 shifting of the strike. Although there is reason to suppose that 

 a good deal of the sedimentary beds was absorbed by the laccolite 

 when it was being intruded, yet the vast central area consisting 

 of granite represents new material that has come in and occupies 

 the space which the sedimentary beds once occupied, and, as the 

 top of the laccolite is only slightly domed, it means that there 

 must have been a vast lateral thrust. It appears that this thrust 



