356 
POrULAR SCIENCE REVIEW. 
seldom rising into hills, they are almost invariably found in a 
basin, which can be proved in many cases to be a synclinal 
not merely of subsequent movement, but of contemporaneous- 
deposition. In other words, the vertical distance between two 
given coal seams or other horizons is greater in the centre than 
on the edges of the coal basin ; the thick coal seams of the cir- 
cumference of the area have divided finger-like towards the 
centre, where the land was subsiding at a quicker rate, and the 
intervening space has been filled in by sedimentary material, 
deposition keeping pace with the rate of subsidence, so that the 
uppermost coal seam covering the whole is contemporaneous 
over the whole area. The Staffordshire thick coal, splitting 
into so many seams, is perhaps the best example of this. In 
the Newcastle field there is a thinning and splitting towards 
the centre of the coals, and thickening of the sediment. This 
is also the case in Lancashire, the measures between the Arley 
and Ince Yard coals thickening towards the centre of the basin 
66 feet per mile for each mile traversed, or from 1,500 to more 
than 2,300 feet. And here the remarkable fact works out that 
at the time of the formation of the Arley seam the rate of sub- 
sidence was the same over the whole field ; but a little later in 
time an axis of greater subsidence came into being, which 
gradually travelled to the north-east during the deposition of 
the whole of the middle coal measures, until it reached the dis- 
trict around Wigan — the area where the celebrated Wigan 
cannel is of greatest purity and thickness. Here vegetable 
matter would reach its most complete condition of maceration, 
and here the cannel-coal vegetable matter in the most extreme 
state of comminution is thickest. Associated with the cannel 
are the scales of large fishes, which, salmon-like, swam through 
the water from the more open sea. With the cannel and at 
various other horizons in the coal-field are thick seams of An- 
thracosia shells, near to our freshwater mussel. Whether they 
represent it or the estuarine condition indicated by the Scrobi - 
cularice , which so constantly occur in the grey clays intercalated 
with the Lancashire mosses, is still a moot point with na- 
turalists. 
The character of the trees found at the base of the Lanca- 
shire mosses varies with the nature of the underlying soil. 
Generally the roots penetrate the grey estuarine or fresh- water 
clay beneath, resembling much in appearance and chemical 
composition the underclay of the coal measures, containing 
the stigmaria-roots of sigillarian tree trunks, found broken and 
scattered in the coal and shale above, just as the trees found 
in the mosses have been broken off at a certain distance above 
the ground. 
Through obstructed drainage, water has accumulated around 
