466 



GEOLOGY OF CAPE COLONY 



which were formed very probably at a considerable 

 distance from the spot where the plants grew, and the 

 alternation of thin layers of coal and silt evidently points 

 to the vegetable matter having been deposited over the 

 floor of the basin in the same manner as the silt. It is 

 this silt which accounts for the high percentage of ash 

 in the Stormberg coals. 



The abundant intrusions of dolerite in the form of 

 dykes and sheets, especially the latter, have had an in- 

 jurious effect upon the coal. The distance through which 

 this influence has made itself felt varies ; the chief effect 

 is the driving off of the more volatile constituents with 

 the development of an anthracitic character culminating 

 in the coking of the coal, which is thus rendered unsale- 

 able. The insertion of a few analyses of the coals taken 

 from the official reports just referred to may be of use 

 in indicating the class of coal to which the Colonial 

 seams belong : 



While the amount of ash is considerable, much higher 

 than in most of the other South African coals, it must 



