Page:South African Geology - Schwarz - 1912.djvu/72

 formed by growth in place or by transportation. In Europe and America most of the coal has formed by growth in place; beneath the coal seams there is found a fireclay, which is nothing but the ancient soil and often contains the roots and stems of the trees from which the coal was formed. In Cape Colony the coal has been formed by the transportation of material from afar, and the seams may lie on shale or sandstone and there may be a complete absence of the fireclay. The Transvaal coal has been formed by growth in place. Transportation coal is usually very shaly, the seams being made up of innumerable smaller seams, each separated by a parting of shale.

A vast time is necessary for woody tissue to turn into true coal, and hence the plants we find in coal, and which produced it, are of extinct kinds. In Europe and America the coal plants were called Lepidodendra, Sigillaria, and Calamites, whereas the coal in the southern hemisphere — Africa, India, &c. — was formed only partly by these, the most abundant plant represented as fossils in the coal formations there being ferns, called Glossopteris. If the vast time has not elapsed, then there is an intermediate stage between coal and wood, called lignite, which is a black or brown substance in which the woody structure can still be made out. Lignite deposits are found in the Cretaceous deposits of Oudtshoorn and elsewhere, but not in sufficient quantity to be commercially valuable. Lignite is worked along the Rhine, but it has to be treated by a special process and turned into briquettes before it can be used for fuel.

Metamorphic Rocks. — In quartzites arid marbles the original sandstones and limestones have simply been hardened or recrystallized. By far the most common