242 



PROF. A. H. GEEEN ON THE GEOLOGY AND 



(Bain) and " Trap -Conglomerate " (Wyley). These names imply 

 that it is of volcanic origin, a point which is by no means certain. 



From its very remarkable nature and from the many and varied 

 opinions, some of them of the wildest and most extravagant character, 

 which have been held as to its origin, it has been a great centre of 

 attraction to all who have paid attention to South- African geology. 

 It is a great mass of breccia and conglomerate of variable character, 

 but very coarse in places. My first introduction to this rock was 

 at Grahamstown, under the guidance of Dr. Atherstone, from whom 

 I received not only the heartiest welcome, but also an amount of 

 information and help which was most useful at the outset of my 

 explorations. The rock here has the following character : — A dark 

 grey, closely grained, hard matrix, thickly set with many small 

 angular fragments and rounded grains of quartz. The larger in- 

 cluded blocks not very numerous, the majority angular or subangular, 

 but some well-rounded pebbles ; the largest fragment I saw, of 

 granite, measured 19 inches by 11 inches, and was quite angular. A 

 whitish granite, rich in quartz, was by far the commonest rock 

 among the fragments, but quartzite and other rocks occur. In many 

 ])laces QO bedding is discernible, but where the rock has been exposed 

 it has a bedded look, parallel bands making their appearance on the 

 weathered face, differing from one another in colour, and some 

 weathering faster than others ; along the outcrop too it weathers in 

 a very peculiar way which suggests bedding. Slabs which look 

 something like rudely shaped tombstones stand up in long parallel 

 rows all along the hillside, and it looks very much as if these were 

 hard beds, and that the spaces between had been occupied by some- 

 thing softer. Under the microscope the matrix is seen to consist 

 of a greenish-black opaque substance, set with transparent fragments 

 of all sizes, from the tiniest dot, only just visible under a power of 

 200 diams., up to bits 1 mm. across, with a few two or three times 

 that size. The fragments are markedlj^ angular and of all shapes, 

 most unmistakable chips. They are very nearly all limpid quartz 

 containing dust and fluid-cavities. The fractured nature of the out- 

 line of many of the quartz-grains comes out most beautifully under a 

 tolerably high power ; but often other parts of the edge in the same 

 grain are smoothed and softened off as if by the action of some solvent. 

 Besides the quartz-chips there are scattered sparingly about bits of 

 plagioclase (one of microcline) and decomposed doubtful chips, some 

 of which are probably felstone, and some may be basic traps*. 



I again saw the Dwyka Conglomerate in two localities, in 

 company with Mr. Thomas Bain, the son of the first African 

 geologist, who inherits his father's geological tastes and enthu- 

 siasm, and who gave me the benefit of his genial companionship 

 and great local knowledge during the last three weeks of my 



* Dr. F. H. Hatch, of the Geological Survey, who kindly examined niy 

 slides, has detected the following minerals in addition to those mentioned in the 

 text : — orthoclase, chlorite, magnetite, saussurite, epidote, mica, garnet, zoisite, 

 augite, and olivine. He also notes that, penetrating the quartz-grains, are 

 hair-like needles of rutile. 



