suppose, for illustration, that a small river has become charged with a supply of sand.  As it gradually approaches the sea, and the current loses its force, the sand is the more sluggishly carried along, until finally it falls to the bottom, and forms a layer of sand there.  This layer increases in thickness until it causes the depth of water above it to become comparatively shallow.  On the shallowing process taking place, the current will still have a certain, though slighter, hold on the sand in suspension, and will transport it yet a little further seaward, when it will be thrown down, at the edge of the bank or layer already formed, thus tending to extend the bank, and to shallow a wider space of river-bed.

As a result of this action, strata would be formed, shewing stratification diagonally as well as horizontally, represented in section as a number of banks which had seemingly been thrown down one above the other, ending in thin wedge-shaped terminations where the particular supply of sediment to which each owed its formation had failed.

The masses of sandstone which are found in the carboniferous formation, exhibit in a large degree these wedge-shaped strata, and we have therefore a clue at once, both as to their propinquity to sea and land, and also as to the manner in which they were formed.

[Illustration:  FIG. 19.—­Productus.  Coal-measures.]

There is one thing more, too, about them.  Just as, in the case we were considering, we could observe that the wedge-shaped strata always pointed away from the source of the material which formed them, so we can similarly judge that in the carboniferous strata the same deduction holds good, that the diagonally-pointing strata were formed in the same way, and that their thinning out was simply owing to temporary failure of sediment, made good, however, by a further deposition of strata when the next supply was borne down.

It is scarcely likely, however, that sand in a pure state was always carried down by the currents to the sea.  Sometimes there would be some silt mixed with it.  Just as in many parts large masses of almost pure sandstone have been formed, so in other places shales, or, as they are popularly known by miners, “bind,” have been formed.  Shales are formed from the clays which have been carried down by the rivers in the shape of silt, but which have since become hardened, and now split up easily into thin parallel layers.  The reader has no doubt often handled a piece of hard clay when fresh from the quarry, and has remembered how that, when he has been breaking it up, in order, perhaps, to excavate a partially-hidden fossil, it has readily split up in thin flakes or layers of shaly substance.  This exhibits, on a small scale, the chief peculiarity of the coal shales.