The formation of shales will now demand our attention.  When a river is carrying down with it a quantity of mud or clay, it is transported as a fine, dusty silt, and when present in quantities, gives the muddy tint to the water which is so noticeable.  We can very well see how that silt will be carried down in greater quantities than sand, since nearly all rivers in some part of their course will travel through a clayey district, and finely-divided clay, being of a very light nature, will be carried forward whenever a river passes over such a district.  And a very slight current being sufficient to carry it in a state of suspension, it follows that it will have little opportunity of falling to the bottom, until, by some means or other, the current, which is the means of its conveyance, becomes stopped or hindered considerably in its flow.

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