62S REPORT — 1891. 



matter belonging to plants, as a basis for proving the original quantity of woody 

 matter concerned in a given quantity of coal, would be difficult to determine, for 

 some of the original mineral constituents have been probably removed by per- 

 colating water. 



Professor Lesley^ has calculated that the Mississippi could not supply by 

 driftage from the forests of its valley in 100,000 years wood enough for one of the 

 Schuylkill anthracite beds ; mineral sediments would also interfere with the 

 results. Under favourable conditions, he adds, tropical forests (Central Africa) 

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

 material. So also the swamps of the ' Sunk Country ' of Arkansas and Louisiana, 

 as well as the ' Great Dismal Swamp ' iu Virginia, for one set of conditions 

 (Lyell) ; and the mangrove jungles in the West Indies and elsewhere for another. 

 Fireclay, underclay, undercliff, underbed, seat-earth, seat-stone, b(jt,tom-stone, 

 spavin, chinch, fake, jiounc in. This is usually a dense clay," but sometimes sandy, 

 and even altogether a hard sandstone (' ganister '). It varies in colour from 

 black to white ; and is from six inches to ten feet or more in thickness. A 

 characteristic feature is its being penetrated in all directions by the stigmarian 

 roots and rootlets of the trees {Sigillaria and Lepidodendi-on) 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 periodical dis- 

 turbances to which the many stages of gradual subsidence gave rise. Every coal- 

 bed (or coal-seam, according to the application of those words to either a simple 

 or compound layer of coal) lies on a more or less distinguishable ' underclay ' ; but 

 this is often omitted to be recorded in coal-mining sections and documents.^ Some- 

 times an underclay forms the roof of a coal ; but it is the seat-earth of a coal 

 lying on it. 



Denudation. — Among the many examples of denudation in the Coal-measures, 

 coal-beds have been washed away from their underclays ; but these latter are so 

 greatly toughened by their contained network of roots that they have more 

 effectually resisted denudation. Both coals and underclays, however, were not 

 unfrequently destroyed, or, at least, deeply and widely channelled by contempo- 

 raneous floods and rivers ; for not onlj' are the ' horses,' 'lows,' and 'washes ' such 

 watercourses, but the occurrence of pebbles of coal and small detrital particles 

 scattered through some of the sandstones are due to similar denudation.'* 



Sir J. W. Dawson, in ' Acadian Geology,' 1868, p. lo9, states: — 'The occa- 

 sional inequalities of the floors of the coal-beds, the sand and gravel ridges which 

 traverse them, the channels cut through the coal, the occurrence of patches of 

 sand, and the insertion of wedges of such material, splitting the beds, . . . are 

 constantly represented in modern swamps and marshes, more especially near their 

 margins, or where they are exposed to the effects of ocean storms or river inunda- 

 tions.' The great thickness of coal and carbonaceous shale in the Albion Coal- 

 measures at Pictou, Nova Scotia, were formed in a depression separated by a 

 shingle bar (conglomerate) from the more exposed flats outside.* 



9. Fossils of the Coal-measures of South Wales. — An examination, or even an 

 enumeration, of the fossils would be much more than we have time for now, 

 whether we took in hand the plants or the animals. 



I. Of the characters of the former^ we have indicated some particulars, such as 



' Manual of Coal, k,c., 1856. 



^ In examining microscopically the ultimate particles of some shales and under- 

 clays, Mr. W. M. Hutchings has discovered that these are composed of a ' micaceous 

 deposit,' in which there is some fragmental mica, but that the mass appears to con- 

 sist mainly of minute, rutiliferous, mica-like flakes, regarded by him as of secondary 

 origin, made from the original components of the stratum. (Geol. Maff., 1890 and 

 1891.) Mr. Hutchings kindly informs me that, of the numerous fireclays which he 

 has examined, several are being used for brick-making. (Letter, May 20, 1891.) 



' De la Eeche, 3fe)ii. Gcol. Surr., vol. i., pp. 173 and 177. 



■* Logan, De la Beche, Buddie, and others. ^ Dawson, Q.J.G.S.. vol. x., p. 46. 



' A useful compendium of our knowledge of coal-plants in 1863, by Professor 

 John Morris, was published in the Proceed. Geol. Assoc, of that date. 



