THE CARBONIFEROUS PERIOD. 165 



Carboniferous period; and our knowledge on this subject is 

 daily undergoing increase. It is not to be supposed, however, 

 that the remains of plants are found solely in the Coal- 

 measures; for though most abundant towards the summit, 

 they are found in less numbers in all parts of the series. 

 Wherever found, they belong to the same great types of vege- 

 tation ; but, before reviewing these, a few words must be said 

 as to the orgin and mode of formation of coal. 



The coal-beds, as before mentioned, occur interstratified 

 with shales, sandstones, and sometimes limestones; and there 

 may, within the limits of a single coal-field, be as many as 80 

 of 100 of such beds, placed one above the other at different 

 levels, and varying in thickness from a few inches up to 20 or 

 30 feet. As a general rule, each bed of coal rests upon a bed 

 of shale or clay, which is termed the " under-clay, " and in 

 which are found numerous roots of plants ; whilst the strata 

 immediately on the top of the coal may be shaly or sandy, 

 but in either case are generally charged with the leaves and 

 stems of plants, and often have upright trunks passing vertically 

 through them. When we add to this that the coal itself is, 

 chemically, nearly wholly composed of carbon, and that its 

 microscopic structure shows it to be composed almost entirely 

 of fragments of stems, leaves, bark, seeds, and vegetable debris 

 derived from land-plants, we are readily enabled to understand 

 how the coal was formed. The " under-clay " immediately 

 beneath the coal-bed represents an old land-surface some- 

 times, perhaps, the bottom of a swamp or marsh, covered 

 with a luxuriant vegetation ; the coal-bed itself represents the 

 slow accumulation, through long periods, of the leaves, seeds, 

 fruits, stems, and fallen trunks of this vegetation, now hardened 

 and compressed into a fraction of its original bulk by the pres- 

 sure of the superincumbent rocks; and the strata of sand or 

 shale above the coal-bed the so-called " roof " of the coal 

 represent sediments quietly deposited as the land, after a long 

 period of repose, commenced to sink beneath the sea. On 

 this view, the rank and long-continued vegetation which gave 

 rise to each coal-bed was ultimately terminated by a slow 

 depression of the surface on which the plants grew. The 

 land-surface then became covered by the water, and aqueous 

 sediments were accumulated to a greater or less thickness upon 

 the dense mass of decaying vegetation below, enveloping any 

 trunks of trees which might still be in an erect position, and 

 preserving between their layers the leaves and branches of 



