THE INTRUSIVE DOLERITES AND ALLIED ROCKS 287 



materials for plants which are set free during the slow 

 decomposition of the rock by the action of the weather 

 and the damp soil. It is only to the east of King 

 William's Town in the country within fifty miles of 

 the coast that full advantage can be taken of the valu- 

 able soils derived from the dolerite ; the outcrops there 

 are no longer marked by krantzes or definite ranges of 

 kopjes, but large areas of that rock are covered with 

 fairly deep soil, and unweathered lumps of dolerite are 

 rarely met with in the soil itself. In the arid central 

 and western districts the soil cannot accumulate rapidly 

 enough to clothe the steep slopes, for it is not held together 

 sufficiently by grass and other plants to resist the occa- 

 sional heavy rains. In the place of the extensive, richly 

 grass-covered plateaux of the east, we find extremely 

 rocky ground sparsely dotted over with small bushes, 

 and supporting grass only for short periods after rain. 



The colour of the dolerite hills is usually dull red, 

 but extraordinarily vivid crimson and yellow patches 

 are often met with, and are due to a lichen growing on 

 the weathered crust of the rock. The most exposed 

 surfaces of dolerite boulders in the drier regions become 

 coated with a very thin film of deep brown or black 

 material, which has often a well-polished appearance. 

 This thin coat seems to be chiefly composed of hydrated 

 oxides of iron derived from the rock immediately be- 

 neath it. Dolerite, however, is not the only rock which 

 behaves thus in the dry parts of the Colony. 



The dolerites do not seem to contain valuable minerals 

 in sufficient quantity. Copper pyrites accompanied by 

 pyrrhotine and carrying a small amount of nickel has 



