162 HISTORICAL PALZONTOLOGY. 
tation ; but, before reviewing these, a few words must be said 
as to the origin and mode of formation of coa/. 
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 
or 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. Asa 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 débris 
derived from /and-plants, we are readily enabled to understand 
how the coal was formed. The “«wnder-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 coalbed 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 
plants brought down from the neighbouring land by streams, 
or blown into the water by the wind. Finally, there set in a 
slow movement of elevation,—the old land again reappeared 
above the water; a new and equally luxuriant vegetation 
flourished upon the new Jand-surface ; and another coal-bed 
was accumulated, to be preserved ultimately in a similar 
fashion. Some few beds of coal may have been formed by 
drifted vegetable matter brought down into the ocean by rivers, 
