Page:South African Geology - Schwarz - 1912.djvu/85

 boss intruded into soft slate, then another type of mountain of circumdenudation results.

Underground Water. — In limestone regions, water sinking through the cracks in the rocks dissolves a certain amount in its passage, and in time the crack is enlarged. On the surface these enlarged cracks are trumpet-shaped, and are called swallow holes. They may be filled with water, when they form peculiar circular pools; or they may be choked up with soil, which is markedly more moist than the surrounding soil. The underground cracks, when enlarged, become subterranean rivers, which drain off all available moisture, and the surface becomes arid and waterless. This type of country is called karst, and is typically developed on the east of the Adriatic, but is also well exhibited in the Kaap plateau in Bechuanaland. The rivers issue on the low ground, and deposit quantities of calcareous tufa or travertine, which represents the material dissolved in the passage through the limestone. Caves and subterranean lakes are similarly formed, the most famous examples of which are the Cango caves of Oudtshoorn.

All rocks are penetrated by water, and the term living rock refers to a rock still in its natural position and from which this water has not been driven off. In the lower depths of the earth's crust this water has an important role in ore formation. But besides this water there is also underground water that can be tapped with bore holes. Mostly this is contained in cracks due to the long heating and cooling which results from the change from summer to winter. In the Karroo these seasonal changes of temperature can be felt down to about 60 to 80 ft.; below, the rock is uncracked,