STRATIFICATION OF A COAL-FIELD. 81 
examination of the section of a coal-pit, is the uniform 
presence of a thick bed of clay beneath every layer of coal ; 
but a still more extraordinary fact remains to be mentioned, 
namely, that a common plant of the coal strata, called Stig- 
maria, (hereafter described, see Lign. 36, 38,) invariably 
occurs, more or less abundantly, in this bed of under-clay, 
although very rarely to be met with in the coal or shale 
above. This phenomenon, long since noticed by Martin, 
Macculloch, and other authors, but whose value was not duly 
estimated till the recent observations of Mr. Logan, (Geol. 
Proc. vol. iii. p. 275,) is also found to prevail throughout 
the Welsh coal formation, which is upwards of twelve thou- 
sand feet in thickness, and contains more than sixty beds 
of coal, and as many of clay with stigmariz; the Appala- 
chian coal-measures of the United States present the same 
characters.* To place this fact before the student in a clear 
point of view, I will describe one of the triple series of beds 
which compose a coal-field. 
1. Under-clay; the lowermost stratum. A tough argil- 
laceous substance, which upon drying becomes a grey friable 
earth: it is occasionally black, from the presence of carbona- 
ceous matter. It contains innumerable stems of stigmarie, 
which are generally of considerable length, and have their 
rootlets or fibres (see Lign. 38) attached, and extending in 
every direction through the clay: these stems commonly 
lie parallel with the planes of the bed, and nearer to the 
top than to the bottom. 
2. Coal. A carbonized mass, in which the external 
forms of the plants and trees composing it are obliterated, 
but the internal structure remains; large trunks or stems, 
and leaves, are rarely distinguishable in it, but the presence 
of coniferous wood in many beds of coal, proves that this 
* See Prof. Rogers, in the Proceedings of the American Geologists, 
p. 453; and Sir C. Lyell’s Travels in America. 
VOL., I. G 
