Petrography of Rocks. 279 



Under the microscope it is seen to consist of an alternation of 

 layers of very line-grained quartzose sediment and lenticular patches 

 and streaks of crystalline calcite. 



The quartzose bands consist essentially of very minute chips of 

 quartz, and probably feldspar, with a good deal of mica, both colour- 

 less and brownish : there is also a considerable amount of black, 

 carbonaceous matter along the bedding planes. Much calcite is 

 present in scattered crystals. The calcareous layers are usually more 

 or less lenticular in shape, and are composed of crystals of calcite 

 (or dolomite) showing secondary twinning. The whole rock shows 

 distinct signs of crushing, and might almost be spoken of as foliated 

 parallel to the original bedding planes. 



So far as the origin of this rock is concerned, two explanations 

 are possible : — 



(a) It may be a fine-grained sediment, containing pebbles 

 of limestone, which have subsequently been squeezed out of 

 shape and flattened by pressure. 



(b) The calcareous matter may have been in the form of 

 spheroidal or ellipsoidal concretion, due to segregation in the 

 original rock, and afterwards recrystallised under pressure, 

 perhaps accompanied by thermal metamorphism to a certain 

 extent. 



All we can definitely say is that the rock is a sediment, containing 

 both siliceous and calcareous matter, which has been subjected to a 

 certain degree of metamorphism, so that it has been crushed and 

 more or less recrystallised. The thermal metamorphism is probably 

 due to the overlying volcanic rocks, while there is nothing to indicate 

 the source of the crushing. 



D. THE GRANITE. ' 



Below this sedimenta];y rock, down to the greatest depth repre- 

 sented in the collection, viz., 2520 feet (215) is found a rather pale, 

 grey granite, which in places shows a somewhat gneissose structure. 

 Tt has already been pointed out that the granite is reached at a con- 

 siderably less depth in the De Beers Mine, indicating either a very 

 uneven surface of denudation, or else intrusion of a very irregular 

 mass. This last supposition is unlikely, since the sedimentary rocks 

 within a few feet of the granite show only a feeble degree of thermal 

 metamorphism, so far as can be judged from the small amount of 

 material at hand. It is much more probable that the upper surface 

 of the granite represents an ancient buried landscape, as in the case 

 of the Mount Sorrel granite and other pre-triassic rocks of the 

 English Midlands. * However, it must be admitted that this is 

 almost pure speculation, founded on very insufficient evidence. 



It so happens that specimens of granite from the De Beers Mine 

 are much less decomposed, and show somewhat greater variety than 

 those from Kimberley, so that they will be treated of more fully 

 later on. 



* Watts, Geogr. Journal, June, 1903. 



