Page:South African Geology - Schwarz - 1912.djvu/121

 dykes intruded into the sedimentary beds, which they have not had sufficient energy to convert into liquid rock. Such a case is presented by the biggest laccolite, bar one, in the world, the Bushveld laccolite in the Transvaal, which is 250 ml. in diameter, and begins a short distance north of Pretoria; the town itself lies on the complex of dykes that surround the central core.

In yet other cases the expansion does not form a solid nucleus at all, but the igneous matter on its way upwards breaks up into innumerable dykes or sheets; this is the cedar-tree type of the Americans. The South African example is incomparably the most gigantic laccolite in existence on the earth; it extends right through the breadth of South Africa from Natal to the Atlantic, a distance of 700 ml., and forms a belt 200 ml. broad, running through the Karroo. Individual sheets are from 200 to 300 ft. thick, and can be followed for scores of miles; they rise in steps one above the other, those on the south inclining gently towards the centre on the north and those on the north inclining gently southwards. The total amount of liquid rock that is shown by the dolerite now exposed on the surface would make a sphere 50 ml. in diameter. In the Great Karroo the dolerite is seen capping the line of escarpment which separates it from the High Karroo, and on top of the plateau one is lost in a maze of sheets and dykes of dolerite in all directions. To the east the dolerite country lies in a region of greater erosion, and in places such as at Cradock or Queenstown the whole country is one succession of towering dolerite-capped hills. Possibly beneath the Central Karroo there lies a solid core of igneous material, and conversely it is possible that before the covering was denuded from off the Bushveld laccolite the sedimentary