THE INTRUSIVE DOLERITES AND ALLIED ROCKS 261 



These dykes then must be regarded as the channels 

 through which the rock composing the sheets flowed. 

 The hard Pre-Karroo rocks must have been rather re- 

 sistant to the invading magma, but the soft and evenly 

 bedded Karroo sandstone and shales could be split up 

 very readily, and the molten material was able to form 

 thick sheets spreading underground over enormous 

 areas. 



The lowest sheet in Calvinia, for instance, certainly 

 extends over an area of 3,000 square miles and probably 

 a third more ; l the lowest sheet north of Hopetown must 

 have been continuous over at least 5,000 square miles 

 without reckoning the area it occupied in the adjoining 

 part of the Orange River Colony. 2 



A considerable number of dolerite dykes penetrate the 

 Ibiquas series in the west of Calvinia, while a few are 

 found in the Karroo beds of the Tanqua Valley, but the 

 main area of the intrusions commences to the north of 

 the Tanqua River. 



In the Dwyka series between the Langebergen 

 (Calvinia) and the Tanqua Valley there is a very exten- 

 sive sheet which stretches, with a few breaks in the 

 northern part of the outcrop, for rather over 100 miles ; 

 it is at places 300 feet thick. This sheet, and indeed all 

 those in the western part of the Colony, tend to rise to- 

 wards the south-east and they traverse higher and 

 higher beds in the same direction. The lowest sheet 

 first appears near the base of the Dwyka conglomerate 

 north of the Oorlog's Kloof River, but at the base of 

 Potkly's Berg East it is in the lower part of the Ecca 

 >G, C.,v.,p. 50. 2 G. C., xi., p, 124, 



