bone or plant tissue, and the successive coats have been laid over the original nucleus concentrically. The bedding of the original rock passes through the septaria, and only when the rock weathers do the septaria become prominent, because of their superior hardness. The septaria usually contract after they have reached a certain size, and cracks form, which are subsequently filled in with pure calcite. The strange forms of these septaria in the Karroo shale and elsewhere has led to their being mistaken for fossil tortoises, &c.
Sometimes iron plays the part of cementing material, the order of events being the same as in the septaria. Then the structure is called a concretionary nodule. Red or yellow nodules of red or yellow ochre are common in the Witteberg rocks.
Coals are due to plant action. In many tropical forests, as in Sumatra, the trees grow, flourish, and die, and an accumulation of rotting vegetable matter forms about their bases. In northern climates moss forms accumulations of peat, which is similar in nature to the accumulations in the Sumatra forest; but the coal seams, showing throughout remnants of Stigmaria, or the roots of the great coal-period trees, Lepidodendron and Sigillaria, prove that most of the coals are formed from the higher plants. If such a mass of rotting vegetation becomes sealed up by mud and sand deposited by the sea which invades the land, then it will slowly alter to coal. In other places, as at the mouth of the Magdalene River, on the north of South America, vast quantities of trunks and branches of trees and other vegetable matter, torn from the forests by storms, are swept out to sea at every flood, and this material, becoming waterlogged, accumulates in the sediment at the bottom of the sea. Coal may thus be
