180 Mr. Bain on the Geology of Southern Africa. 



Gneiss, 



which never occurs but in contiguity with the granite ; for it seems always to 

 pass into clayslate when beyond the influence of the plutonic rock below. Section 

 No. 4, PI. XXL, shows the invasion of the latter on the ancient sedimentary 

 deposits, where its veins are seen to penetrate the laminated rocks, whilst huge 

 detached masses of granite are entangled in the gneiss, and fragments of gneiss 

 remain isolated in the granite. 



Gneiss occurs at Platte Klip at the foot of Table Mountain, Hottentots Holland, 

 and near Little Brak River in the district of George. I consider it to be con- 

 temporaneous with the 



Clayslate, 



which is very extensively developed both in the southern and western parts of the 

 colony, and is of great thickness. This formation contains rocks of various kinds, 

 and presents a great diversity of aspects. At Cape Town extensive quarries are 

 worked in it for building-purposes, and at Robben Island, in Table Bay, the rock 

 is cut into square flags for pavements, grave-stones, &c., taking a smooth face 

 and cutting very freely. It is there of a bluish-black colour, but is devoid of slaty 

 cleavage, as are, I believe, all the other schists of the colony. In many parts it 

 passes into mica-slate, as at Swartland, Tulbagh, Slaryhoek, Worcester, and many 

 other parts. In some places it is completely decomposed into a soft clay, of 

 various colours, — yellow, blue, pink, white, and brown, as at Bain's Kloof, where, 

 in a tunnel which I perforated, through a thickness of 400 feet, I never required 

 even the aid of a crowbar in the excavations (except through some massive quartz- 

 veins), the whole being worked by the pickaxe and spade. 



The clayslate, like the granite, never rises to any great height ; but is generally 

 found to constitute all the valleys of elevation in the western and southern parts of 

 the colony, extending as far east as Gamtoos River. 



The laminse or strata, when visible, are always at high angles, frequently vertical 

 and contorted, and often entirely overturned, having evidently suffered much from 

 lateral compression. It is quite impossible to calculate the thickness of the clayslate. 

 I have made many extensive excavations in these rocks, but never saw the least sign 

 of a fossil in them. Hence I conclude that they are non-fossihferous. Numerous 

 quartz-veins traverse them in all directions. Their strike, like that of all the older 

 rocks, is generally from S.E. and N.W, to E.S.E and N.N.W. ; and, as far as my 

 researches go, they are destitute both of metals and minerals. 



Lying unconformably on the clayslate is a formation of great thickness and 

 extent, which, from its geological position, though not at all from its physical 

 attributes, I consider to be the equivalent of 



