236 



PHYSIOGRAPHY. 



[chap. 



when examining the great coal-field of South Wales, that 

 eacl\ bed of coal is supported by a layer of shale known as 

 unde?--ciay or seat-earth, as shown in Fig. 65. Whatever the 

 number of seams of coal — and in some cases they are very 

 numerous — there is always just the same number of under- 

 clays. Moreover these underclays usually contain such 

 bodies as that figured in Fig, 66, which are never found 

 in the roof of the coal. Such objects had long been 

 known to geologists under the name of stigmarice^ but 

 though it was clear they represented some part of a plant, 

 their precise nature long remained an enigma. At length a 



Fig. 66. — Stigmariajicoides; a coal measure fossil. 



railway cutting through the Lancashire coal-field exposed 

 half a dozen trees resting upon a seam of coal, but sending 

 their strong roots downwards into the under-clay, where 

 they ramified in all directions and gave off rootlets. It was 

 found by Mr. Binney that these roots were nothing but the 

 well-known stigmaria, and that the characteristic stigmata, or 

 pits, were not leaf-scars, as had been suggested, but points 

 from which rootlets had been given off. The - stigmariae 

 passed upwards into fluted tree-stems, which are not un- 



1 Stigmaria, from ffriy/xa, sli^ma, 

 scars left by the rootlets. 



' a mark : " in allusion to the 



