Yol. 54.] WITWATERSRAND AND OTHER DISTRICTS IN S, TRANSVAAL. 91 



With regard to the volcanic rocks, they are of a basic character, 

 consisting chiefly of verj- fine-grained to compact aphanitic rocks of 

 the basaltic or diabasic type. The following description of thin 

 sections, prepared from some of the more typical specimens in a 

 collection of these volcanic rocks made by me, will give an idea of 

 their microscopic structure. A section of a specimen from a basic 

 flow near Potchefstroom shows under the microscope the character of 

 an altered basalt, consisting of a plexus of microlitic felspar and 

 granular augite. Its vesicular cavities are filled with secondary 

 quartz. A compact aphanitic rock from the Witwatersberg is seen, 

 under the microscope, to consist of fragments and granules of 

 augite and magnetite in a groundmass of minute felspathic needles. 

 A section of a specimen taken from an outcrop near Lindique 

 discloses a characteristic diabase with ophitic structure. A dolerite 

 from the Witwatersberg shows well-crystallized hypersthene, with 

 well-marked pleochroism, striped felspar in subordinate quantity, 

 and a little pleochroic brown mica in small shreds and patches. A 

 quartz-diabase from the Yaal River consists of plagioclase, augite, 

 and quartz, the microstructure being distinctly granophyric. 



Summary or the Cape System. 



Summarizing the above information, we find that the Cape 

 System, as developed in the Southern Transvaal, has an aggregate 

 thickness of, roughly, 50,000 feet, the estimated thickness of the 

 different formations being as follows : — 



Feet. 



Magaliesberg and Gatsrand Series 16.000 to 20,000 



Dolomite Formation 6000 to 8000 



Black Eeef Formation 20 to 50 



Klipriversberg Amygdaloid 5000 to 6000 



Witwatersrand Beds 11,000 to 15,000 



Hospital Hill Series 8000 to 10,000 



lY. The Karoo System. 



The northern fringe of the formations belonging to this system, 

 so largely developed in the Cape Colony and the Orange Free State, 

 is represented by the Coal Measures ^ at Yereeniging and the district 

 bordering on the Yaal south of Heidelberg, by the Boksburg, 

 Brakpan, and Springs coal areas and by numerous small outlying 

 patches. In all these places the coal, which is associated with 

 shales, is overlain by a well-bedded coarse sandstone, which forms an 

 excellent freestone, and underlain by grits, breccias, and fireclays 

 corresponding to the gannister of the English Coal Measures. 



The coal seams are usually of considerable thickness, and embrace 

 varieties of gas-, coking-, smithy-, and steam-coal. 



A general section at Yereeniging (for which I am indebted to 

 Mr. Noble, Consulting Engineer to Messrs. Lewis & Marks), 

 which may be taken as fairly representative, shows the following 

 beds : — 



^ The ' High Yeldt Beds' of Penning {op. jam cit. p. 457). 



