268 
THE GEOLOGIST. 
bog, that, through subsidence, became covered with water to a sufficient 
depth to admit of land plants being washed from the shores; and these, 
becoming loaded with mineral matter, sank and were imbedded in the 
mud : the lighter portions of the plants, such as the stems, leaves, and 
parts of the bark, would remain a longer time suspended, but would 
at last sink and be buried in the accumulating silt, afterwards to form 
those carbonaceous spots we now find intercalated in the stone. 
The overclays of the Coleford High Delf seam, and many of the other 
veins, contain numerous impressions of ferns (Pecopteris), as well as 
some fine trunks of Sigillariae. By far the greater number of the 
latter, however, are found in the sandstone beds, where they have 
usually their roots (stigmariae) attached. I have often seen, in tolerable 
abundance, in the roof of the Coleford High Delf seam, the casts of a 
shell resembling a Cyclas. 
The coal-measures of the Forest of Dean present us, perhaps, with 
fewer examples of faults and dislocations of strata, the presence of 
igneous dykes, or of subterranean disturbance on a large scale, than 
any other British coal-field of similar size. We have, however, in this 
field certain interruptions to the continuity of the coal-seams, which are 
locally termed faults, but which have derived their existence from 
channels cut by running water, similar to those examples of erosion still 
to be observed in extensive bogs, and to pristine irregularities in the 
Burface of the bed on which the vegetable matter subsequently formed 
into coal was deposited. The latter kind of fault is exceedingly 
common, and often completely " cuts out " the coal, manifesting its 
origin, however, by the undisturbed bedding of the coal, ending 
Bmoothly on either side of the interruption, as well as by the very gentle 
inclination with which its outline rises from the floor. The subjoined 
diagram illustrates an example from a colliery I am working at 
Buardean. In this case, the fault was formed by a bank, the lineal 
extension of which is yet undiscovered, but which was upwards of forty 
yards in average breadth, and formed, in all probability, a small spit of 
land which projected into the morass. 
Lign. 3.— Coal-Seams " cut out " by a " horse," Ruardean. a. Clod-top (roof) j b, nnderclay (sole) ; 
c, 1, 2, 3, 4, coal-seams. 
