v.j ON THE FORMATION OF COAL. 99 



" Indications of spore-cases are rare, except in certain coarse shaly 

 coals and portions of coals, and in the roofs of the seams. The most 

 marked case I have yet met with is the shaly coal referred to as con- 

 taining Sporangites in my paper on the conditions of accumulation of 

 coal (Journal of the Geological Society, vol. xxii. pp. 115, 139, and 

 165). The purer coals certainly consist principally of cubical tissues 

 with some true woody matter, and the spore-cases, &c., are chiefly in 

 the coarse and shaly layers. This is my old doctrine in my two papers 

 in the Journal of the Geological Society, and I see nothing to modify it- 

 Your observations, however, make it probable that the frequent clear 

 spots in the cannels are spore-cases." 



Dr. Daw son's results are the more remarkable, as the 

 numerous specimens of British coal, from various locali- 

 ties, which I have examined, tell one tale as to the 

 predominance of the spore and sporangium element in 

 their composition ; and as it is exactly in the finest and 

 purest coals, such as the "Better-Bed" coal of Lowmoor, 

 that the spores and sporangia obviously constitute almost 

 the entire mass of the deposit. 



Coal, such as that which has been described, is always 

 found in sheets, or " seams," varying from a fraction of 

 an inch to many feet in thickness, enclosed in the sub- 

 stance of the earth at very various depths, between beds 

 of rock of different kinds. As a rule, every seam of 

 coal rests upon a thicker, or thinner, bed of clay, which 

 is known as " under-clay." These alternations of beds 

 of coal, clay, and rock may be repeated many times, 

 and are known as the " coal-measures ; " and in some 

 regions, as in South Wales and in Nova Scotia, the 

 coal-measures attain a thickness of twelve or fourteen 

 thousand feet, and enclose eighty or a hundred seams 

 of coal, each with its under-clay, and separated from 

 those above and below by beds of sandstone and shale. 



The position of the beds which constitute the coal- 

 measures is infinitely diverse. Sometimes they are tilted 

 up vertically, sometimes they are horizontal, sometimes 

 curved into great basins ; sometimes they come to the 



H 2 



