Page:South African Geology - Schwarz - 1912.djvu/73

 metamorphic rocks are the crystalline schists, which are shales or mud not only recrystallized but the substance of the mud changed to various crystals, quite different from those in the substance of the original rock. The term schist implies the fact that the rock is fissile or will split easily, and this character is due to the minerals of the schists being arranged with their flat faces all more or less parallel.

The commoner schists are chlorite or green schist, talc schist, black mica or biotite schist, and white mica or sericite schist.

Basic igneous rocks give hornblende schists which are rich green in colour.

Schists with felspar are called gneisses.

Meteorites are rocks that fall from the sky; they make themselves visible as they enter the earth's atmosphere by burning, and are known as shooting stars. By far the larger number burn entirely, and the ash thus formed falls as an impalpable powder as cosmic dust. The pure snowfields of the Arctic are covered with this fine dust. Some fall to the earth and are found to be of several kinds, varying from pure metallic ones to pure stony ones. The iron ones are very noticeable when they lie on the surface of the earth, and may be proved to be truly meteorites by polishing a surface and etching it with nitric acid, when an intricate system of crossing lines is seen to form, these being due to the crystalline nature of the nickel-iron alloy of which the meteorites are composed. The stony meteorites are mostly composed of ultra-basic rocks similar to the rocks in the Kimberley mines, but with characteristic rounded patches in them. Some meteorites consist partly of stone and partly of iron. It has been estimated that about a