PEOF. T. EtJTERT JONES — OX COAL. 93 



accumulated so fast, or was so dro-^ned in "water, that the hydrogen 

 and oxygen could not all es(;ipi> from the carbon, and hydro- 

 carbons -were formed by recombinations. Hence the presence of the 

 alternate planes (and edgewise of streaks) of bright and dull black 

 materials in common coal, f Dawson. J 



Under favourable conditions, tropical and subtropical forests 

 (such as those of Central Africa, Erazil, and elsewhere), and 

 coast-swamps (Florida, Guiana, India"), would supply good and 

 sufficient material. ISo also would the swamps of the " Sunk 

 Country" of Arkansas and Loiisiana, as well as the " Great Dismal 

 Swamp " in A'irginia, for one set of conditions, and the mangrove 

 jungles in the West Indies and elsewhere for another. 



6. 'The Materials of the Coal-measures. — Different vaneties of the 

 old coal are due to the fact that some coal contains more charcoal, 

 some more hydro-carbon, and some more mud than others. Hence 

 certain localities have given characteristics to the local coal. 



In some collieries the miners are able to work at the coal under 

 a hard roof of shale, which is muddy material that was deposited 

 in the water, having been brought down from the higher ground, 

 and forming layers, sometimes many yards thick. This shale 

 contains a great many remains of plants, and sometimes of shells. 



After this accumulation of shale, generally large quantities of 

 sand were deposited ; and this formed sandstone, very useful for 

 builtUug and paving. A layer of pure clay was often formed on 

 these sand-beds ; and this is used for making fire-bricks. 



Fireclai/^ underclay, underdiff., underhed, seat-earth, seat-stone, 

 'bottom-stone, spavin, clunch, fake, ov pouncin, is usually a dense clay, 

 but sometimes sandy. It varies in colour, and is from six inches 

 to ten feet or more in thickness. It is penetrated in all directions 

 by the Stigmarian roots and rootlets of the trees [Sigillaria and 

 Lepidodendron) that grew on it when it was the soil of the coal- 

 forest, having been slowly deposited by the quiet, shallow, muddy 

 waters that succeeded the deposition of shale or sandstone by 

 waters with stronger currents ; these last terminating one of the 

 periocHcal disturbances to which the many stages of gradual 

 subsidence gave rise. Fragments of Scorpions and Eurypterids 

 occur plentifully in some of the "old soils" (fireclays). The 

 former, being land-animals, and probably adapted to a hot (or, at 

 least, warm) climate, are among the most interesting of the coal- 

 fossils. 



All of the above mentioned beds, layers, or strata were repeated 

 thousands of times. 



In Xova Scotia there are about 80 coal-seams in 14,570 feet of 

 shales, clays, and sandstones. In the coal-field of South Wales 

 there are 12,000 feet of coal-seams, shales, sand, etc., and nearly 

 100 seams of coal worth working. 



The order and thickness of the strata belonging to the coal-field 

 of South Wales, as given by Sir Archibald Geikie,*" are (for 

 Glamorganshire) : — 



* 'Textbook of Geology,' 2nd edit., 1885, p. 742. 



