C— GEOLOGY. 



53 



are the uutlerclays and gannisters — ' seat-earths ' to use their common 

 denominator. These are the clayey or sandy beds which underlie coal- 

 seams, or have stood in the relation of soil to vegetation insufficient in 

 amount to form coal. There are many more of these old soils than 

 there are coal-seams. 



Both underclays and gannisters possess physical and chemical 

 characteristics which separate them from all other deposits. Physically 

 they are destitute of signs of lamination, and they are traversed by 

 carbonaceous markings often to be identified with certainty as the roots, 

 rootlets, and rhizomes of plants, usually lycopodiaceous. These have 

 pierced through and through the material, obliterating any traces of 

 bedding which may once have been there. The chemical peculiarities 

 of the old soils are the absence of alkalies, the rarity of calcium com- 

 pounds, and the low percentage of iron. These features also can be 

 ascribed to abstraction of mineral substances by plant activity, arid 

 perhaps in part to leaching by passage of water charged with organic 

 compounds arising from the decay of vegetable tissues. 



In some beds — but never in the seat-earths — molluscan remains 

 occur. They are of two contrasted groups. Shells of Carbonicola, 

 Anthracomya, and NaiadUes are commonly found in packed masses in 

 which, as is commonly the case in fresh-water muds of our day, the 

 lack of variety is compensated for by a great abundance of individual 

 specimens. Beds containing these fossils are usually argillaceous, but 

 they sometimes constitute important beds of ironstone. The marine 

 bands offer the remains of a much more varied fauna — not usually 

 with the same superabundance of individuals. The fossils include 

 Pterinnpecten, Pseudamusium, Lingula, Ortlioceras, goniatites, and 

 many other genera — obviously a marine assemblage. Sometimes the 

 tine blue shales thus characterised are accompanied by hard limestone. 



In the bright-coloured measures belonging to the Staffordian division 

 limestones of peculiar texture and contents are found — commonly called 

 Spirorbis Limestones. Their texture is usually smooth and fine; occa- 

 sionally they enclose angular fragments of a similar limestone, as 

 though a deposit had been shattered in situ by some agency, and deposi- 

 tion of like material had then been renewed. 



To all these add coal-seams and the series is complete. 



The Constituents of Coal-seams. 



We must now turn to a more minute examination of the coal itself. 

 Any seam of ordinary house coal — such, for example, as the Silkstone 

 seam — will present us, if we look closely, with three substances, 

 agreeing in a carbonaceous character and in a modicum of combustibility, 

 but otherwise very different. The first constituent is a black, lustrous, 

 dense material— the typical coal. This is disposed in apparent bedding 

 with some appearance of regularity, though an individual layer can 

 rarely be traced as much as three or four feet, and is never continued 

 for many more. It presents in great perfection a close and regular 

 cleavage perpendicular to the bedding. This cleavage is the cleat, of 

 which I shall have more to say presently. 



The second constituent, which is rarely altogether absent though 



