Petrography of Rocks. 



.87 



to the Beer Vley volcanic series.* The overlying quartzites and 

 shales are also considered to be of pre-Cape age; the " melaphyre," 

 which next succeeds, is compared to the Zeekoe Baard amygdaloid, 

 which is also of pre-Cape age. Here there occurs a great gap in the 

 series, since the whole of the Cape formation is absent, and the thin 

 representative of the Dwyka conglomerate, of Lower Karroo age, rest 

 directly on the pre-Cape volcanic rocks. The basalt is in all 

 probability of Karroo age. 



The general results of the foregoing petrographical study of the 

 rocks surrounding the diamond-bearing pipes, and the nomenclature 

 adopted may be summarised in the following table, which also gives 

 the maximum and minimum thickness observed in different cases. 



The nomenclature here used is purposely left as general as 

 possible, since it is quite impossible in most ca.ses to say deiinitely 

 from an examination of hand-specimens and slices alone whether a 

 given rock is of a hypabyssal or volcanic nature, and, in point of 

 fact, the distinction between these two classes is a very artificial one. 

 Large masses of lava often assume distinctly hypabyssal characters, 

 on account of somewhat slower cooling, and narrow dykes often con- 

 sist of rock having typical volcanic structure, e.g., some of the great 

 augite-andesite dykes of the north of England. 



The evidence in this case is meagre, and no stress can be placed 

 on it, but it may at any rate be said that none of the igneous rocks 

 seen in these sections, exclusive, of course, of the granite, show any 

 features which are inconsistent with volcanic origin. I'hey can, 

 however, hardly all be part of one great series, since they must cover 

 an enormous interval of time, owing to the great gap in tlie succession 

 below the Karroo series, if we accept the correlation of these rocks 

 given by Rogers. The acid and intermediate series may be connected 

 although they show no points of resemblance, but the basic series 

 belongs to a much later period. 



The diagram on page 288 shows the results arrived at in a 

 graphic form. It is compiled partly from the published works of Mr. 

 Gardner Williams, and partly from information supplied with the 

 collection of specimens. 



* Geology of Cape Colony, p. 341. 



