358 Q. W. Buhnan — On Underclays. 



(3) " The Bottom coal usually rests on a bed of fire-clay some 

 feet in thickness. In many instances, however, this is wanting, and 

 the coal rests on hard sandstone, the change from one material to the 

 other being sometimes very abrupt" (p. 209). 



And underclay is at times so associated with sandstone as to 

 suggest for it a similar origin as to place and manner. For example, 

 in the Survey Memoir of the Yorkshire Coal-field we have the 

 following section : 



FT. IN. 



Coal 9 



Underclay ivitJi bands of sandstone ... ... 3 4 



Black shale 1 



Underclay ... ... ... ... ... 1 8 



On the other hand underclay may be mixed with sandstone, as 

 in the case of a white sandstone 3 ft. 6 in. thick, met with in 

 a boring at Seaton Carew (Durham), and separated from a coal- 

 seam above by 4 in. of dark-blue shale (Notes on the Seaton Carew 

 Boring. J. W. Bird. Nov. 8th, 1888). 



A.nd in whatever place and manner underclays were formed, 

 vegetable matter — probably drift — must in some cases have had 

 access. For fragmentary masses of coal — in the form of strings and 

 veins — are at times found in it, as in the following section from the 

 Survey Memoir of the Yorkshire Coal-field : 



FT. IN. 



Coal 3 6 



Uuiiderclay and Coal-veins 8 



Sandy underclay ... ... ... ... 1 6 



Sandy shale ... ... ... ... ... 1 9 



Sandstone 2 



Underclay and Coal-veins ... ... ... 4 1 



Underclay 2 (p. 519). 



It is difficult to draw any distinction as to mode of origin between 

 such cases and those in which the coal appears to play the role of 

 " partings " in a seam of underclay, as in the following section : 



Coal 



Underclay 



Coal 



Underclay 



Coal 



Underclay 



Coal 



(Survey Memoirs, Yorkshire Coal-field, p. 167). 



Fi'om this we pass naturally to the case of a Coal-seam in which, 

 the underclay partings attain the same thickness as the intervening 

 coal, as in the following section of a portion of the "Fire-clay" coal 

 of the South Staffordshire Coal-field : 



Little coal 



Fire-clay ... ... 



Coal 



Fire-clay 



Coal 



(Survey Memoir, p. 202). 



