[Illustration:  FIG. 18.—­Sigillarian trunks in current-bedded sandstone.  St Etienne.]

As the most apparent of the rocks of the system are sandstone, shale, limestone, and coal, it will be necessary to consider how these were deposited in the waters of the carboniferous ages, and this we can best do by considering the laws under which strata of a similar nature are now being deposited as sedimentary beds.

A great proportion consists of sandstone.  Now sandstone is the result of sand which has been deposited in large quantities, having become indurated or hardened by various processes brought to bear upon it.  It is necessary, therefore, first to ascertain whence came the sand, and whether there are any peculiarities in its method of deposition which will explain its stratification.  It will be noticed at once that it bears a considerable amount of evidence of what is called “current-bedding,” that is to say, that the strata, instead of being regularly deposited, exhibit series of wedge-shaped masses, which are constantly thinning out.

Sand and quartz are of the same chemical composition, and in all probability the sand of which every sandstone in existence is composed, appeared on this earth in its first solid form in the shape of quartz.  Now quartz is a comparatively heavy mineral, so also, therefore, will sand be.  It is also very hard, and in these two respects it differs entirely from another product of sedimentary deposition, namely, mud or clay, with which we shall have presently to deal when coming to the shales.  Since quartz is a hard mineral it necessarily follows that it will suffer, without being greatly affected, a far greater amount of wearing and knocking about when being transported by the agency of currents and rivers, than will a softer substance, such as clay.  An equal amount of this wearing action upon clay will reduce it to a fine impalpable silt.  The grains of sand, however, will still remain of an appreciable average size, and where both sand and clay are being transported to the sea in one and the same stream, the clay will be transported to long distances, whilst the sand, being heavier, bulk for bulk, and also consisting of grains larger in size than grains of clay, will be rapidly deposited, and form beds of sand.  Of course, if the current be a violent one, the sand is transported, not by being held in suspension, but rather by being pushed along the bed of the river; such an action will then tend to cause the sand to become powdered into still finer sand.

When a river enters the sea it soon loses its individuality; it becomes merged in the body of the ocean, where it loses its current, and where therefore it has no power to keep in suspension the sediment which it had brought down from the higher lands.  When this is the case, the sand borne in suspension is the first to be deposited, and this accumulates in banks near the entrance of the river into the sea.  We will