chap. tii. Sandstone Formation. 169 



most imposing feature in the geology of Southern 

 Africa. The strata are in many parts horizontal, and 

 attain a thickness of about 2,000 feet. The sandstone 

 varies in character ; it contains little earthy matter, 

 but is often stained with iron ; some of the beds are 

 very fine-grained and quite white ; others are as com- 

 pact and homogeneous as quartz rock. In some places 

 I observed a breccia of quartz, with the fragments 

 almost dissolved in a siliceous paste. Broad veins of 

 quartz, often including large and perfect crystals, are 

 very numerous ; and it is evident in nearly all the 

 strata, that silica has been deposited from solution in 

 remarkable quantity. Many of the varieties of quartzite 

 appeared quite like metamorphic rocks; but from the 

 upper strata being as siliceous as the lower, and from the 

 undisturbed junctions with the granite, which in many 

 places can be examined, I can hardly believe that these 

 sandstone-strata have been exposed to heat. 1 On the 

 lines of junction between these two great formations, I 

 found in several places the granite decayed to the 

 depth of a few inches, and succeeded, either by a thin 

 layer of ferruginous shale, or by four or five inches in 

 thickness of the re-cemented crystals of the granite, on 

 which the great pile of sandstone immediately rested. 



Mr. Schomburgk has described 2 a great sandstone 

 formation in northern Brazil, resting on granite, and 

 resembling to a remarkable degree, in composition and 

 in the external form of the land, this formation of the 

 Cape of Good Hope. The sandstones of the great plat- 

 forms of Eastern Australia, which also rest on granite, 

 differ in containing more earthy and less siliceous 



1 The Eev. W. B. Clarke, however, states, to my surprise 

 (' Geolog. Proceedings,' vol. iii. p. 422), that the sandstone in some 

 parts is penetrated by granitic dikes : such dikes must belong to an 

 epoch altogether subsequent to that when the molten granite acted 

 on the clay -slate. 



2 ' Geographical Journal,' vol. x. p. 246. 



