49] 
the coal-beds, we appeal to the three same intermitting and alternate 
processes of subsidence, drift, and vegetable growth; the subsidence 
being in the former case to a depth below the level of the sea, in 
the latter case to a depth which left the last-formed strata in a po- 
sition to become the site of vast swampy flats and shallow lagoons. 
In both cases intermitting accumulations of the earthy materials of 
the strata over the subsided districts are referred to the transport of 
sand and mud by powerful land-floods over areas which by subsi- 
dence had acquired a place that. made them receptacles of the de- 
tritus of distant mountains; as we now see vast sheets of sediment 
transported from the Rocky Mountains and spread over the great 
flats and vast estuaries of the Red River, the Missouri, and the 
Missisippi. The regions on which these ancient alternations of salt- 
water and fresh-water deposits were going on, must in the mean 
time have presented extensive surfaces that were periodically os- 
cillating between small distances above and below the level of the 
sea. 
The concentric rings of growth which may be counted in a trans- 
verse section of the large coniferous trees whose roots are found 
resting on the upper surface of a coal-bed, may be quoted as evi- 
dence of the time during which it was fixed in this its place of 
growth ; and as such trees may probably be found on the surface of 
many successive beds in the section of a coal-field, each stage of 
trees affords a chronometer by which we may calculate the number 
of years that intervened between the growth of each bed of coal. 
In the Neweastle collieries, after the excavation of the coal, short 
trunks of trees drop down frequently from the roof of the mine, 
leaving vertical cavities, which the miners call pot-holes; these trees 
probably grew upon the surface of the vegetable mass by which the 
coal has been formed; and the occasional assemblage of large num- 
bers of cones and seed-vessels of the same species, €.g. of Lepido- 
strobus and Trigonocarpum, upon one spot, seems to indicate that 
they dropped into their present place from the trees on which they 
grew. 
Should the above hypotheses be correct, we may expect to find 
corresponding differences of organic structure on microscopic exa- 
mination of the vegetable remains in the lower and upper portions 
of many beds of coal; and the attention of observers may at this 
time be profitably directed to the examination of thin slices of coal, 
carefully selected from different regions of the same bed, for the 
purpose of ascertaining whether differences exist between the com- 
ponent vegetables of the upper and lower regions of individual 
strata, sufficiently obvious and constant to justify us in referring the 
lower region of certain strata to a sub-aqueous, and the upper re- 
gion to a sub-aérial origin. Should an entire bed of coal exhibit 
no other vegetable structure than that of Stigmaria, it may be 
inferred that these plants had not so far filled up the lagoon in 
which they grew, as to convert it to a sub-aérial swamp, before fresh 
floods of water from the land overwhelmed these sub-aqueous ve- 
getables with sand and silt. Should we find another coal-bed with- 
