ECONOMIC GEOLOGY 465 



Hoek, and at Sterkstroom. The uppermost is the 

 horizon of the Molteno seam, 300 feet above the Indwe 

 seam ; to it may be referred the coals at Molteno and 

 Cyfergat, Bamboes Spruit, and the thin seams at the 

 Gala Pass, Olyve Fontein a little to the south-west of 

 Aliwal North, and Gubenxa in Elliot. 



The layers of coal seldom exceed twelve inches in 

 thickness (in the Guba seam there is one about twenty- 

 five inches thick), but as several usually occur alternat- 

 ing with thin bands of black shale it is possible to 

 extract from three to four feet of coal in mining 

 operations. The working costs are enormously in- 

 creased by the presence of these thin shaly layers. 



On all three horizons these composite seams appear to 

 occupy a number of detached areas, in between which 

 the coal is either replaced by shale or else is entirely 

 absent. In most cases this is due to non- deposition of 

 carbonaceous material, but sometimes to erosion of the 

 matter deposited contemporaneous erosion, a pheno- 

 menon which is seen in thousands of cases throughout 

 the Karroo beds. At Indwe the upper layers of coal and 

 shale are in places missing, and the surface thus denuded 

 is overlain by massive sandstone with pebbles at its base. 



The coals of the Molteno beds 1 are usually laminated 

 and contain very thin streaks of shale ; they are coals 



1 For fuller details see the following : 



E. J. Dunn, Report on the Stormberg Coal Fields, Cape Town, 1878 ; 

 F. W. North, Report on the Coal Fields of the Stormbergen, Cape Town, 

 1878 ; A. H. Green, Report on the Coal Fields of the Cape Colony, Cape 

 Town, 1883 ; W. Galloway, Report on tlie Coal Deposits in tlie Indwe 

 Basin and Stormberg Range of Mountains, Cape Town, 1889 ; and G. C., 

 viii., ix. and x. 



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