Page:South African Geology - Schwarz - 1912.djvu/69

 direction of the current changing, the top of the heap is washed away and the new deposit of sand is laid upon the older at a different angle. Current-bedded or false-bedded sandstones are characteristic of all the sandstone formations in South Africa, notably the Table Mountain Sandstone.

Where desert conditions prevail, the rocks may be split up by contraction and expansion due to variations of temperature, and the sand resulting contains fragments of all the minerals of the original rock quite unaltered. If the original rock is a granite, the sandstone containing the broken debris is called an arkose; otherwise such mixed sandstones are called grits.

In warm climates the shores abound in animal life, and dead shells are washed up in great abundance. These are dragged backwards and forwards by the tides until they are broken into fragments small enough to be carried by the wind. The dunes resulting from the accumulation of such sand will not be siliceous, but will be calcareous. The South African coast is surrounded by aeolian or windborne sand of this nature, and the fragments of shells being easily soluble in water containing a little carbon dioxide, the rainwater when it sinks into the dunes dissolves a certain amount, which is deposited when the water evaporates and cements the grains together; hence fairly firm limestones are produced which are used for building purposes. Quarries of this consolidated shell sand have been worked at Saldanha Bay and Bathurst; the Bluff at Durban consists of this material.

Muds, or argillaceous rocks, are made up of aluminium silicate or kaolin, derived from the disintegration of felspars. They are usually coloured grey, blue, or black with impurities, and weather to a clay.