When the river enters a large body of water, such as the ocean or a lake, in losing its individuality, it loses also the velocity of its current, and the silt tends to sink down to the bottom. But being less heavy than the sand, about which we have previously spoken, it does not sink all at once, but partly with the impetus it has gained, and partly on account of the very slight velocity which the current still retains, even after having entered the sea, it will be carried out some distance, and will the more gradually sink to the bottom. The deeper the water in which it falls the greater the possibility of its drifting farther still, since in sinking, it would fall, not vertically, but rather as the drops of rain in a shower when being driven before a gale of wind. Thus we should notice that clays and shales would exhibit a regularity and uniformity of deposition over a wide area. Currents and tides in the sea or lake would tend still further to retard deposition, whilst any stoppages in the supply of silt which took place would give the former layer time to consolidate and harden, and this would assist in giving it that bedded structure which is so noticeable in the shales, and which causes it to split up into fine laminae. This uniformity of structure in the shales over wide areas is a well ascertained characteristic of the coal-shales, and we may therefore regard the method of their deposition as given here with a degree of certainty. There is a class of deposit found among the coal-beds, which is known as the "underclay," and this is the most regular of all as to the position in which it is found. The underclays are found beneath every bed of coal. "Warrant," "spavin," and "gannister" are local names which are sometimes applied to it, the last being a term used when the clay contains such a large proportion of silicious matter as to become almost like a hard flinty rock. Sometimes, however, it is a soft clay, at others it is mixed with sand, but whatever the composition of the underclays may be, they always agree in being unstratified. They also agree in this respect that the peculiar fossils known as _stigmariae_ abound in them, and in some cases to such an extent that the clay is one thickly-matted mass of the filamentous rootlets of these fossils. We have seen how these gradually came to be recognised as the roots of trees which grew in this age, and whose remains have subsequently become metamorphosed into coal, and it is but one step farther to come to the conclusion that these underclays are the ancient soils in which the plants grew. No sketch of the various beds which go to form the coal-measures would be complete which did not take into account the enormous beds of mountain limestone which form the basis of the whole system, and which in thinner bands are intercalated amongst the upper portion of the system, or the true coal-measures.