THE INTRUSIVE DOLERITES AND ALLIED ROCKS 281 



together with the abundance of intrusions in the undis- 

 turbed portions, is very suggestive. 



The age of the intrusions can be fixed with some prob- 

 ability. Boulders of dolerite are found in the Embotyi 

 conglomerate of the Pondoland coast, probably of Upper 

 Cretaceous age, while along the Buffalo River a small 

 patch of Upper Cretaceous beds rests upon a sheet of 

 dolerite. The Uitenhage conglomerates have hitherto 

 been found only at a distance from dolerite outcrops, 

 so the absence of boulders from these conglomerates 

 throws no light on the matter. Seeing that the doler- 

 ites followed the Stormberg volcanics (Lower Jurassic) 

 fairly closely and that an enormous amount of denuda- 

 tion must have taken place subsequently, but prior to 

 Upper Cretaceous times, it appears most likely that the 

 dolerites were intruded during the Jurassic Period. 



At the contact of the dolerite sheets and dykes with 

 the sedimentary rocks there is generally a noticeable 

 hardening of the latter through a distance varying with 

 the thickness or width of the intrusion. Shales and 

 mudstones are changed to lydianite or hornstone, a dark 

 hard flinty-looking rock which breaks with a conchoidal 

 fracture ; the typical hornstone is only a few inches in 

 thickness, but sometimes the metamorphism extends 

 over a considerably greater distance. 



In a few cases the heat has been so intense that the 

 sediment at the contact has been fused ; the product so 

 formed is a black vitreous material which under the 

 microscope is found to be full of minute crystals of 

 cordierite, tiny octahedra of magnetite and sometimes 

 corroded grains of quartz. 



