182 ME. G. W. LAMPLTJGH ON THE [May I907, 



country into basalts on the one side and sandstones on the other, 

 leaving no link between them. One is hampered also in attempting 

 an estimate by the uncertainty as to the thickness of the basalts, 

 regarding which we know only that it must exceed 1000 feet ; but 

 by how much, it is impossible at present to conjecture (p. 195). 

 Considering their total disappearance from the high ground on the 

 opposite side of the fault together with the probable depth to 

 which their base is sunk on their own side, it is not likely that 

 their downthrow at the fault is less than 2000 feet ; and it may be 

 very much more. The fact that all the basalts should have perished 

 from the upthrow side and that the country should have been planed 

 down to its present condition on both sides, denotes considerable 

 antiquity for the fault. 



It will be noted that this great dislocation throws inward towards 

 the centre of the continent, and may therefore have lent aid in the 

 construction of the interior basin of the South African plateau. 



(3) The Batoka Basalts. 



Under any conditions, large areas of plateau-basalt are somewhat 

 monotonous deserts to the stratigraphical geologist, and such to 

 me the Batoka Basalts proved. During our long treks over their 

 surface, I gleaned but scanty information on many essential points ; 

 for which, perhaps, the necessity of following a definite route 

 through the wild country may be partly responsible. Thus I 

 nowhere saw their true base, nor did I find any recognizable vent 

 from which these old lavas were poured ; and, what is still more 

 regrettable, I did not light upon any intercalated sediments such as 

 the previous descriptions had led me to expect. 



The Batoka Basalts present the usual characteristic of basic lavas, 

 in maintaining great uniformity of composition over wide areas. 

 All the varieties that were found may be included under the term 

 olivine-dolerite, used in its wider sense. When massive, they 

 form dark-blue heavy rocks, varying somewhat in the size and 

 relative abundance of their crystalline constituents, and still more 

 in the degree of development of a vesicular or amygdaloid al 

 structure and in the composition of the amygdules. Stratigraphically 

 these basalts present the usual trap or step-like features, due to the 

 intercalation of thick bands of the more massive rock with thinner 

 bands of less durable vesicular breccias which mark the under and 

 upper surfaces of individual lava-flows. This structure is often excel- 

 lently brought out by weathering in the walls of the canons and along 

 the broken edges of the plateau, the hard bands protruding in bold 

 scarps or ' krantzes,' and the slaggy breccias breaking down into 

 inclines. 



bridge (fig. 3, p. 177) — the shattered sandstones seemed slightly to overhang 

 the basalt, though with very little departure from verticality. I saw no 

 indication of overthrusting, however, and do not think that the dislocation is 

 actually a ' reversed fault,' though the basalts may perhaps in places have sunk 

 slightly under the edge of the sandstones. 



