TRANSACTIONS OP THE SECTIONS. 51 



feet, separating the coal into two distinct seams. At Alkrington the two Beat Mines 

 are worked together, but to the S.E. a parting of fire-clay appears, which gradually 

 increases in thickness, and at Oldham, 3 miles distant, the two Beat Mines are worked 

 10 yards apart; other instances of subdivision are known, all of them taking place 

 towards the S. and S.E. Independently of the tendency to divide, many seams di- 

 minish in thickness till they become evanescent; this is chiefly observable in the 

 lower division of the coal-field, and in the simple seams six beds which have been 

 worked in that series, give decisive evidence of this fact. The best examples are the 

 caking coal of Rochdale and the Foot mines, beds known by various names in different 

 parts of the country, but easily identified by the remarkable nature of their floor, 

 which is a hard crystalline stone, called Ganister, full of Stigmaria ficoides, and em- 

 ployed as a material for mending roads. At Dulesgate, near Todmorden, the upper 

 or " Ganister coal " is 5 feet 8 inches in thickness, and the Foot coal, about 12 

 yards below it, is 7 inches thick; the author has traced these seams about 11 miles 

 to Quarlton, and ascertained that the Ganister coal gradually diminishes in thick- 

 ness 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 where a thin seam is found on a thick argil- 

 laceous floor full of Stigmatise, it is certain to become workable if followed. The 

 floors are of three kinds— the fire clay, which is the most abundant; the warrant, 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 Ganister, before no- 

 ticed. 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 bot- 

 tom. 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 Todmorden, 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 supported the vegetation which 

 now constitutes the coal. The remains of bivalve shells and fishes in the cannel beds, 

 prove that they were formed under water; but in the Lancashire coal-field no re- 

 mains cf fishes or shells have yet been found in the coal, nor is there any indication, 

 either by admixture of sand or silt in the seams of coal, to show that they were 

 drifted into the places they now occupy by rapid currents of water. The occurrence 

 of forests of large trees standing upright on the seams, the pure vegetable matter 

 composing the coal itself, with scarce any admixture of foreign ingredients, the posi- 

 tion of the coal upon a rich alluvial deposit well adapted to sustain a luxuriant vege- 

 tation, seem to prove that, in most instances, the vegetable matter forming it grew 

 upon the spot where the coal is now found ; whilst the splitting and alterations in 

 the thickness of the seams themselves, show that the surface was most probably sub- 

 ject to frequent subsidence. 



Statement of the Fossils ichich have been discovered in the several members of 

 the Carboniferous or Mountain Limestone of Ireland, with a view to shoio 

 the Zoological identity of the whole Series, together with a Comparison of the 

 Fossils which occur in the Mountain Limestone of Ireland with those which 

 have been obtained from the same Scries in Great Britain, and also icith the 

 Fossils of North and South Devon, illustrated by Maps, Sections, Draw- 

 ings and Specimens. By Richard Griffith, F.G.S. 

 The substance of this communication had already been printed by Mr. Griffith for 



distribution amongst the members of the Association; it was accompanied by a cata- 



e2 



