380 SURFACE QUARTZITES 



siliceous matrix enclosing the grains of quartz that 

 are often visible without the aid of a magnifying glass. 

 The quartzites generally enclose many small irregularly 

 shaped cavities, which are sometimes lined with minute 

 crystals of quartz, or with the chalcedonic form of silica. 

 The original quartz grains in the rocks are at places 

 converted into bipyramidal crystals by the addition of 

 new quartz in crystalline continuity with the quartz of 

 the grain. By the mutual interlocking of the new 

 quartz added to all the grains in the originally sandy 

 portion of the rock, the loose sands have become in- 

 tensely hard quartzites in which the original grains are 

 no longer recognisable without the use of a microscope 

 and thin sections of the rock, when the outlines of 

 some of the grains can be seen within the new growth 

 of quartz ; the quartz deposited round any one sand 

 grain interlocks closely with that round the neighbour- 

 ing grains. Good examples of these quartzites may be 

 seen in any of the south-western Divisions. They often 

 appear above the soil as rounded polished surfaces, due 

 to the weathering out of the rock along irregularly dis- 

 posed vertical joints, which leave a massive lump of rock 

 in their interstices. On the hilltop near the road from 

 Swellendam to the bridge over the Buffeljagt's Eiver 

 the quartzite has been quarried for building purposes ; 

 the bridge piers are made of it. As a rule, however, the 

 rock is too intractable and too variable within short dis- 

 tances to be worth quarrying, although it is certainly 

 a very durable stone. Near Grahamstown the surface 

 quartzites appear in the Sugar Loaf Hill and on the 

 terrace to the north of it mentioned on a previous page, 



