G. W. Bulman — On Underdays. 357 



Interesting examples of the independent occurrence of coal and 

 underclay are also met with in the Carboniferous rocks of Scotland, 

 where in not a few cases they are separated by beds of sandstone. 

 The following sections from the Survey Memoir of the Geology of 

 Edinburgh (32 Scotland, 1861) may be taken as examples: 



(2). 



(p. 106.) 



(p. 100.) 



Underclays thus occurring without coal are accounted for by Sir 

 Henry de la Beche as follows (Memoir, South Staffordshire Coal- 

 field, p. 338) : 



" As will be readily understood, even all traces of a coal above 

 a Stigmaria bed may be absent, either from the carbonaceous matter 

 having been removed by the stream or current of water which 

 deposited new matter, such as sand, above it, or from the conditions 

 not having been so far advanced as to permit the Stigmaria bed or 

 soil to be coated over with such carbonaceous matter." 



But denudation by streams of Carboniferous vegetation can scarcely 

 have been sufiScieutly uniform over large areas to account for the 

 entire absence of coal over Stigmarian clays ; such denudation would 

 rather cut channels and hollows in the coal. It would, moreover, 

 have left marks of denudation in the underlying clay. 



And even in those cases in South Wales — mentioned in a note on 

 the same page of the Memoir — where the carbonaceous matter has 

 been "entirely removed, and even channels cut in the supporting 

 Stigmaria beds," it is difiScult to believe that the deposit of vegetable 

 matter could disappear so entirel}'' and leave no trace of its presence. 

 Sir H. de la Beche speaks of the erosion having taken place when 

 the coal was " unconsolidated " ; but the carrying away of a dense 

 mass of vegetation firmly rooted in clay would require currents of 

 considerable force. Still there is no difficulty in understanding 

 denudation cutting channels in the coal — like the "Horse" in the 

 Forest of Dean — but denudation which will neatly slice off" a layer 

 of coal from its supporting underclay is another matter. 



Other cases occur in which the same seam of coal rests partly 

 on an underclay, and partly on sandstone. The following three 

 examples are from the Memoir of the South Staffordshire Coal-field : 



(1) " There is often above it a bed of fireclay or clunch a few 

 feet in thickness, supporting the sulphur coal, but that is frequently 

 absent, and the coal rests directly on the rock " [a sandstone, known 

 as the New Mine Coal rock] (p. 195). 



(2) "The upper measure is generally fire-clay or clunch, supporting 

 the fireclay coal, and varying in thickness from 2 to 10 feet. This, 

 however, is sometimes wanting, and the fire-clay coal rests directly 

 on a ' strong rock ' or hard sandstone " (p. 203). 



