296 
SILUKIA. 
[Chap. XII. 
edges ; and hence we can apply to the latter a physical explanation of their me- 
thod of formation which is quite inapplicable to the former. In short, the origin 
of coal in the younger of these accumulations may be very well accounted for by 
the depression of low woodlands and jungles beneath fresh water, followed by 
subsequent elevations and depressions — phenomena which cannot be admitted 
into our reasoning so long as we refer to the marine conditions which prevailed 
in the older Carboniferous era, many of the strata of which must have been 
accumulated in deep seas. 
In Britain, however, there is no such abrupt severance in the middle of this 
natural series ; and yet we there see the same changes from marine or estuary 
conditions to a terrestrial state as in those regions which have undergone a great 
physical disturbance. Thus, in the thick overlying Coal-measures of Newcastle 
and Durham, we lose all traces of marine life, and can recognize only huge accu- 
mulations of lacustrine or fluviatile origin. In South Wales, on the contrary, 
where the Coal-measures are estimated to attain the great thickness of 12,000 
feet, and one hundred coal-beds are, it is said, intercalated at various levels, 
we have undeniable evidence of alternating marine conditions *. At the same 
time, each of these coal-seams has immediately beneath it a band of clay or sandy 
shale, called 'under-clay/ abounding in Stigmaria, or roots of Sigillaria, one 
of the plants from which coal has been generated. For this last important fact 
science is chiefly indebted to Sir William Logan, who, when assisting Sir H. De 
laBeche in mapping the South- Welsh Coal-basin, demonstrated that the 1 under- 
clay ' of the miner had been the soil of a primeval marsh or jungle. (See also 
Mammat's ' Geological Facts,' &c. 1834, p. 73.) The woodcut in the beginning 
of the Chapter is intended to convey a general idea of the nature of the wet 
and swampy tracts in which the vegetation of this period flourished. 
The comparative rarity of true dicotyledons or forest-trees in this flora, due 
perhaps in part to the rapid decomposition of vegetable bodies in a moist warm 
climate t> is as remarkable as the extraordinary uniformity in the families of 
Plants of which it was composed. These consisted chiefly, as before said, of 
Lepidodendra, Sigillarise, Calamitacess, and Ferns, with some curious extinct 
Pine-trees, some of which resembled the living Araucaria of Norfolk Island. 
As many of these are found in the roof of the coal, and even in the coal it- 
self J, there can be no doubt that, in most of these cases, the mineral resulted 
from the decomposition and fossilization of the Plants which grew in exten- 
sive marshy jungles. It may fairly be inferred that this conversion of vege- 
tables into coal took place at a period in the formation of the crust of the earth 
when very different physical conditions prevailed, and when a warmer and more 
equable (though probably not a hot) climate pervaded our islands, as well as la- 
titudes far to the north and south of them. The view which supposes many and 
successive subsidences of vast swampy jungles beneath the level of the waters 
best explains how the different vegetable masses became so covered by beds of 
sand and mud as to form the sandstone and shale of such coal-fields. The theory 
of oscillation, however, or of the subsidence en masse of ancient marshes and their 
* Marine Shells have been found associated with X Few persons, who are attentive observers of 
some of the lowest beds of coal which have been the fuel they consume, will have failed to detect 
worked in the South -Welsh Coal-field ; Bevan, the forms of plants in the coal itself. Gopperthas, 
British Assoc. Report, Trans. Sect. 1858. See also indeed, demonstrated, by microscopic examination 
Mr. Salter's appendix to the ' Iron-ores of South (and Witham in part anticipated him, ' Observa- 
Wales,' Geol. Surv. Memoirs, 1861. tions on Fossil Vegetables,' 1833), that the vege- 
t See Hawkshaw, Proc. Geol. Soc. vol. iii. p. 269. table fibres and tissues of all the families of the 
The reader will also do well to consult Prof, plants of this era are to be detected in the coal 
Lindley's notice of his experiments in decompo- itself. In some layers all the plants are Sigillariae, 
sing vegetable bodies (see Lindley and Hutton's in others Calamites, in others Ferns. (Quart. 
' Fossil Flora of Great Britain,' vol. iii. pp. 1-12). Journ. Geol. Soc. vol. v. part 2, Mem. p. 17.) 
