Basalts, granites, and such rocks do not contain coal; the coal measures in which the seams of coal occur are, generally speaking, limestones, fine sandstones, and shales, that is to say, rocks which in their origin were deposited under water. In detail almost every seam has some individual peculiarity, but the following represents two types of typical seams. In many cases, below the coal, the limestone or sandstone rocks give place to fine, yellow-coloured layers of clay, which varies from a few inches to many feet in thickness and is called the “underclay”. This fine clay is generally free from pebbles and coarse débris of all kinds, and is often supposed to be the soil in which the plants forming the coal had been growing. The line of demarcation between the coal and the clay is usually very sharp, and the compact black layers of hard coal stop almost as abruptly on the upper side and give place to a shale or limestone “roof”; see fig. 13, layers 5, 6, and 7. Very frequently a number of small seams come together, lying parallel, and sometimes succeeding each other so rapidly that the “roof” is eliminated, and a clay floor followed by a coal seam, is succeeded immediately by another clay floor and another coal seam, as in fig. 13, layers 10, 11, and 12. The relative thickness of these beds also varies very greatly, and over an underclay of seven or eight feet the coal seam may only reach a couple of inches, while a thick seam may have a floor of very slight dimensions. These relations depend on such a variety of local circumstances from the day they were forming, that it is only possible to unravel the causes when an individual case is closely studied. The main sequence, however, is constant and is that illustrated in fig. 13. The second type of seam is that in which the underclay floor is not present, and is replaced either by shales or by a special very hard rock of a finely granular nature called “gannister”. In the gannister floor it is usual to find traces of rootlets and basal stumps of plants, which seem to indicate that the gannister was the ground in which the plants forming the coal were rooted. The coal itself is generally very pure plant remains, though between its layers are often found bands of shaly stone which are called “dirt bands”. These are particularly noticeable in thick seams, and they may be looked on as corresponding to the roof shales; as though, in fact, the roof had started to form but had only reached a slight development when the coal formation began again. [Illustration: Fig. 14.—Diagram of Coal Seam with Gannister Floor, in which are traces of rootlets _r_, and of stumps of root-like organs _s_]