Notes on the Geology of the Drakensbergen. 421 



dolerite cliff can be seen forming the sides of the canon, which there 

 has a depth of 2,000 ft. (aneroid measurement). In this and the 

 few other places where it can be well seen in section, the dolerite 

 appears to lie as fiat as the sandstone on which it rests. Measured 

 from the sandstone floor to where the first bed of amygdaloid covers 

 it, the dolerite in the Tugela district does not average more than 

 500 ft. in thickness, but at the Bushman's Pass and Giant's Castle 

 end it is thicker — about 700 ft. The greater the distance of the 

 dolerite from the Berg and the covering amygdaloid, the thinner 

 it is. Ten miles from the mountains it is only a few feet thick, and 

 at greater distances it has been entirely removed, but there can be 

 no doubt that its extent was formerly enormous. 



The lowest portion of the dolerite sheet contains a great quantity 

 of agates, calcedony, crystals, and numerous other forms of quartz,* 

 similar to the pebbles found in the Vaal Eiver near Kimbeiiey, 

 together with occasional quartz veins. I saw these in the dolerite 

 near the Berg only, but not on the ridges over four or five miles 

 distant. But on the spurs two or three miles away the ground is 

 covered with them at and below the level wiiere they occur. At one 

 place I noticed similar pebbles in the sandstone below, just at the 

 point of contact where it has been altered into a quartzite. This 

 alteration is but skin deep, however, extending to little more than 

 eighteen inches or two feet. 



There are a number of large dykes of this same dolerite running 

 nearly parallel to each other near the Bushman caves, and these 

 can be easily followed from ridge to ridge for miles. At the Mont 

 aux Sources end of the range they are not so regular, but still very 

 numerous. These may now fill up the fissures through which the 

 dolerite sheet was erupted. In these dykes no agates were noticed. 



In conclusion, I estimate the remains of this sheet to extend in 

 Natal in scattered, uneroded survivals over an area sixty miles by 

 fifteen at least. How far it may extend beyond Giant's Castle in 

 the direction of Cape Colony I do not know. The foregoing descrip- 

 tion is the result of observations made during thirteen different 

 ascents of the mountains or spurs, the larger number being in the 

 Bushman's Eiver Pass and Giant's Castle district. 



The cave sandstone, which lies below the great dolerite sheet, is 

 a compact, hard, gritty rock, weathering with a rough surface, which 



* I did not, although on the look out for them at Pilani, Orange Free State, 

 notice these agates, &c., in the dolerite, but at Harrismith I remember an 

 alluvial deposit of them on the flats 1,800 ft. below the level of the top of 

 Plaatberg. 



