494 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



Ras-Las, Nine-feet, Bydylog in different parts of the field; its varia- 

 tions may be taken as typical. At most localities, it is double with 

 a variable parting of clay, but there are definite partings, without 

 inorganic deposit, separating benches, differing in the character of 

 their coal. At some localities, it is troubled by "nips," the shale 

 roof disappearing and the underclay becoming sandstone, while the 

 coal thins away. In some districts there are considerable areas 

 where the coal is so poor that it is not worth mining ; yet, in most 

 localities, it is a thick bed and yields excellent coal. 



" Washouts " are by no means infrequent. The Ras-las is miss- 

 ing at one place in northern Monmouth. In the Ebbw Valley of the 

 same county, a great washout was encountered, extending 1,200 

 yards and causing removal of a section, 116 yards thick, with three 

 coals, Three-Quarters, Black and Yard. The Ras-las has been 

 washed out for not less than a mile in the Sirhowy Valley ; in the 

 Bargoed-Taff Valley it has been rendered almost worthless at many 

 places by wedge-shaped masses of shale, cutting down to or nearly 

 to the base of the seam, but not below it. These resemble channels 

 of rivulets, filled with mud before deposit of the overlying rocks. 

 Local deposits of coal are not unusual. De la Beche^* saw, near the 

 village of Bagelly in southern Pembrokeshire, some irregular masses 

 of stone coal. One, semi-oval, is 140 yards long, 40 wide and 10 

 deep ; four others of similar type were observed. Such coals seem 

 to characterize the Millstone Grit, since all are local. Cannel is not 

 abundant ; it may be on top or midway in a coal seam, but it is 

 always in contact with the coal. A seam, one foot thick, was seen 

 at one place, but it appears to be local. 



Strahan^^ has emphasized the plasticity of shale when between 

 more resistant rocks as shown at some places in the Neath Valley. 

 There as well as in Cynon Valley near an overthrust fault, the Nine- 

 feet coal has a layer of shale pressed in to close wrinkles ; the coal 

 has become schistose, weathering into plaquettes, with razor edges, 

 slickensided and very brittle. 



The lower shales and sandstones of the Lower Coal Series have 



9* H. T. de la Beche, " On Geology of Southern Pembrokeshire," Trans. 

 Geol Soc, II., Vol. 2, 1829, p. 19. 



P5A. Strahan, Part IV., p. 16; Part V., p. 65. 



