504 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



Top Coal rises until it is 29 feet above the Bottom, thence descends 

 until the two benches are again in contact : the same condition is 

 shown in a second drift as well as in a neighboring colliery. This 

 phenomenon is not rare ; it has been observed in several seams 

 within the Yorkshire field and geologists have reported its occur- 

 rence in other fields. Kendall thinks it is due to the filling of a 

 channel with sand or clay, over which the swamp extended. The 

 originally level top of the in-swept material is now convex while the 

 originally convex bottom is flat. He conceives that during conver- 

 sion of the peat into coal, the thin borders of the enclosed mass 

 adjusted themselves to the changing thickness of the organic mate- 

 rial until the upper surface became convex and the bottom flat. 

 The existence of the gravel deposit has been proved along its west 

 side for about 5 miles; the mass has been crossed by drifts at two 

 places, which show a width of not less than 1,200 feet. Existence 

 of such " splits " is known in the Silkstone Coal at many places, but 

 these have not been connected by continuous workings. Kendall 

 feels justified in asserting that the splits mark courses of ancient 

 streams. 



Limestone rarely occurs in the Yorkshire field, the prevailing 

 rocks being sandstones, shales and underclays. The mollusks are 

 mostly Anthracosia and Anthraconiya, which are at many horizons, 

 but undoubted marine forms are present in some thin black shales. 

 The sandstones vary from conglomerate to fine-grained. The 

 coarser rocks are irregularly bedded and in many cases they resem- 

 ble huge heaps, thinning away in all directions. But there are such 

 deposits, especially in the Millstone Grit, of vast extent and showing 

 little variation in thickness or character. Lepidodendron and Cala- 

 mites casts are not rare. Nearly all sandstones, coarse and fine 

 alike, are false-bedded, often with marked current- or cross-bedding, 

 and the finer sandstones frequently are ripple-marked. Shales may 

 be sandy, blocky, passing into sandstones, or they may be argilla- 

 ceous ; sometimes they are black, passing occasionally into cahnel. 

 Underclay, known as Spavin or Seat Earth, is usually clay, always 

 iinstratified, never splits into layers, breaks into irregular blocks and 

 always contains Stigmaria, with rootlets ramifying in every direc- 

 tion. " Many instances have been observed where fossilized trunks 



