450 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



that the igneous rock is everywhere continuous beneath it. 

 Moreover, it is thickest about the centre and thins out in 

 every direction against the surface of the intrusive, not 

 excepting the direction of the confined valley by which the 

 creek leaves the mass. Here for a short distance there 

 is no actual shale to be found, though there is every reason 

 to suppose that a connecting neck of shale may have 

 existed until comparatively recent times. On its outer 

 edges the intrusive mass abuts nearly vertically against 

 the shale, as will be shown later. In shape, therefore, the 

 igneous mass is a rather elongated irregular oval, its upper 

 surface being depressed in the centre. 



In composition the main mass of the igneous rock 

 resembles the olivine-gabbros and dolerites (diabases), 

 though, having only about forty-two per cent, of silica, it 

 is more basic than their average by about four or five per 

 cent. Its composition and association show, however, 

 that, though poor in alkalies, it is in reality an essexite, 

 similar to the essexites of Brandberg and Solvsberg. The 

 essential constituents of the main Prospect rock are a 

 violet-brown titaniferous-augite and an acid labradorite in 

 about equal proportions (roughly 36% each), olivine (about 

 10%), ilmenite and magnetite (about 13%); and as acces- 

 sories, sometimes increasing in importance, occur biotite 

 and apatite. The outer envelope of the intrusive mass is 

 a very dark compact grey rock, having the appearance and 

 microscopic characters of basalt. Going inwards from the 

 edge it passes gradually into a rock having the appearance 

 of a very fine-grained dolerite, and the grainsize continu- 

 ally increases until it becomes fairly uniform in the central 

 parts of the mass. Here the rock, when fresh, has a 

 speckled black and white appearance, the individual crystals 

 of augite and felspar being easily visible in the hand speci- 

 men; or, where partly decomposed, it has the chloritic 

 green colour so well known in weathered dolerites. Im- 



