Page:South African Geology - Schwarz - 1912.djvu/67

 or again with chlorite, calcite, or zeolites. The agates and other minerals are deposited by the water which oozes through the rock, and which in doing so absorbs certain substances; these substances are freed from solution as the water enters the cavities of the amygdules. Basalt with amygdules is called an amygdaloid.

Melilite basalt is a basalt without felspar. It consists of a titanium compound, ilmenite or perofskite, and olivine and augite, both of which consist of iron, magnesia, and silica. It is an ultra-basic lava, which occurs in the chimneys of many volcanoes, especially in South Africa. The olivine usually absorbs water and turns into a soft greenish or bluish material called serpentine, the principal constituent of blue ground. Altered ultra-basic lavas are the matrix of the diamond.

Sedimentary rocks are divided into sub-aqueous and sub-aerial rocks that is to say, those that have been laid down under water or on dry land, like blown sand. Sub-aqueous rocks are again classed according to the nature of the water under which they are deposited; most of them are marine sediments with remains of marine shells and other animals contained in them. Estuarine deposits are those formed under brackish water, and contain mixtures of the remains of both fresh water and marine organisms. Freshwater deposits are those laid down under freshwater lakes or in rivers, and contain remains of animals and plants that live in fresh water. The limestones and salt deposits are sometimes separated from the sands and muds, which are called mechanical deposits, the limestones being classed as deposits due to organic precipitation, and the salt and gypsum beds are called deposits of chemical precipitation.

Sands or Arenaceous Sediments. — By far the larger