C. Lyell on the origin of Coal-fields. 37 
transported over alluvial plains to a considerable distance from 
one or more ranges of mountains. The partial intercalation of 
brackish water-beds at certain points is equally consistent with 
the theory of a delta, the lower parts of which are always ex- 
posed to be overflowed by the sea even where no oscillations of 
level are experienced. 
The purity of the coal itself, or the absence in it of earthy par- 
ticles and sand throughout areas of very great extent, is a fact 
which has naturally appeared very difficult to explain if we at- 
tribute each coal-seam to a vegetation growing in swamps, an 
not to the drifting of plants. It may be asked how during river 
inundations capable of sweeping away the leaves of ferns, and 
the stems and roots of Sigillarie and other trees, could the waters 
fail to transport some fine mud into the swamps? One genera- 
tion after another of tall trees grew with their roots in mud, and 
after they had fallen prostrate and had been turned into coal were 
rass. : 
n the ancient coal of the South Joggins in Nova Scotia, many 
of the underclays show a network of Stigmaria roots, of which 
Some penetrate into or quite through older roots which belonged 
to the trees of a preceding generation. Where trunks are seen 
in an erect position buried in sandstone and shale, rooted Sigil- 
larice or Calamites are often observed at different heights in the 
enveloping strata, attesting the growth of plants at several suc- 
cessive levels, while the process of envelopment was gong on. 
In other cases there are proofs of the submergence of a forest 
