4972 
out any Stigmaria, and interspersed through its whole vertical ex- 
tent with Calamites and other sub-aérial plants, indicating a swampy 
soil, we may conelude that the vegetables which formed this bed of 
coal grew upon humid and swampy flats adjacent to lagoons ; and 
that whilst the latter were accumulating beneath their shallow waters 
the materials of a future bed of coal, formed exclusively of the 
aquatie Stigmaria, the adjacent flats were simultaneously accumula- 
ting materials destined for a similar funetion from the sub-aerial 
swamp-plants of the same era. But in the compound ease of coal 
formed by the conversion of a shallow lagoon into a morass, we 
should find in the lower portion, next aboye the fire-clay, no other 
plants than the aquatic floating Stigmaria, and in the upper region 
of the same bed no traces of Stigmaria, but many kinds of sub- 
aerial plants ; whilst in its middle region we should discover a con- 
tact of aquatic with sub-aérial plants. 
We may explain the frequent occurrence of erect trees immedi- 
ately above the upper surface of a bed of coal, as in the cases we 
have spoken of near Bolton and Chesterfield, by supposing the roots 
of these trees to have found support and nutriment in the entangled 
remains of other plants which had preceded them on the same spot, 
as the Scotch firs grow in peat without touching any subsoil ; but 
cases of trees thus standing erect are comparatively rare exceptions 
to their ordinary state of prostration, caused either by decay or tem- 
pests, or by the violence of the eurrenits that submerged and buried 
with sand and silt the morasses in which they grew. 
Fragments and large stems of trees that are found truncated at 
both ends, and inclined in al! directions in thick beds of sandstone, 
like the coniferous trees at Craigleith and Newhaven, near Edin- 
burgh, seem to have been torn from their native bed and drifted 
with the sand to the place in which they are now imbedded. 
Mr. Logan and Mr. L..L. Dillwyn have discovered pebbles or 
rounded fragments of coal in certain grit beds of the coal forma- 
tion, from’ which we learn that some of the older beds of coal had 
assumed an indurated state before the deposition of the more re- 
cent strata of this great formation, the total thickness of whieh in 
South Wales'is 12,000 feet. At Penclawdd, on the Bury river near 
Swansea, Mr. Logan first found, in 1839, a rounded pebble of can- 
nel-coal in a bed ‘of clay; he subsequently discovered that in the 
Pennant grit of Kilvey Hill, near Swansea, there are many con- 
glomerate beds containing pebbles of coal, intermixed with sand 
and pebbles of ironstone, and very rarely with boulders of granite 
and mica-slate. The pebbles are chiefly of common bituminous 
coal; two only have been found composed of cannel-coal, the only 
seams of which known in the lower coal-measures are 2000 feet 
below the Pennant grit. Mr. Logan believes that coal-pebbles oe- 
cur throughout the whole mass of the Pennant sandstone, the thick- 
ness'of which is 3000 feet, but he has seen no such pebbles in the 
lower coal-measures. 
Mr. Buddle has lately found similar pebbles of coal in the Pen- 
nant grit of the Forest of Dean. 
