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 Sigillarise. 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 

 surface 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 

 smoothly 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 

 Ruardean. 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. 



JLigu. 8— Coal-Seams cut out " by a " horse," Ruardean. a, Clod-top (roof) ; b, underclay (sole) : 

 c, 1, 2, 8, 4, coal-seams. 



