292 



THE GEOLOGIST. 



and ascertained that the Gannister coal gradually dimmishes 

 in thickness to one inch, while the Foot coal increases to two 

 feet, the floors retaining the same character throughout. 



III. Coal Floors. The stratum on which the coal rests is 

 always carefully noticed by practical miners, who believe that 

 w^here a thin seam is found on a thick argillaceous floor full 

 of Stigmariae, it is certain to become workable if followed. 

 The floors are of three kinds — the^re clay, which is the most 

 abundant ; the ivarrant, a clay mixed with a larger amount of 

 silica occurring frequently ; and the rock floors, of which but 

 two instances are known, namely, the floor of the Featheredge 

 coal at Walmersley, which is a rough quartzose sandstone, 

 and the Gannister, before noticed. The latter is merely a 

 fine grained admixture of silica and alumina, varying from 8 

 inches to 2 feet in thickness, always graduating into a fine 

 fire clay at its bottom. All the floors, with the exception of 

 the rock floor of the Featheredge coal, contain Stigmaria 

 ficoides, from the thin seams of the Ardwick limestone, to the 

 two seams in the millstone grit of Gauxholme, near Tod- 

 morden, a thickness of nearly 1600 yards; all the fifteen 

 floors of the Manchester coal-field contain it, and at least 69 

 beds in the middle and lower divisions. The Stigmaria 

 generally occurs with its leaves attached, and in all instances 

 of true floors without any intermixture of other plants. 

 These facts seem to indicate that all the deposits were 

 formed under nearly similar conditions ; the roofs and floors 

 were evidently very quietly deposited, and formed a strong 

 clay, well adapted for the growth of the vast masses of 

 vegetable matter required for the formation of the coal seams. 

 The absence of alkalies in the clay of the floors, might be 

 expected from the exhausting properties of plants, and 

 seems to strengthen the supposition that these beds sup- 

 ported the vegetation which now constitutes the coal. The 

 remains of bivalve shells and fishes in the cannel beds, prove 



