V ON THE FORMATION OF COAL 147 



always found in sheets, or " seams," varying from 

 a fraction of an inch to many feet in thickness, 

 enclosed in the substance 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 gi^eat basins; 

 sometimes they come to the surface, sometimes 

 they are covered up by thousands of feet of rock. 

 But, whatever their present position, there is 

 abundant and conclusive evidence that every 

 under-clay was once a surface soil. Not only do 

 carbonized root-fibres frequently abound in these 

 under-clays ; but the stools of trees, the trunks of 

 which are broken off and confounded with the bed 

 of coal, have been repeatedly found passing into 

 radiating roots, still embedded in the under-clay. 

 On many parts of the coast of England, what are 



