STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 493 



thickness is much less than was estimated by the earHer survey. 

 The Lower Carboniferous Hmestones and shales are without coal. 



The Upper Coal Measures, consisting of shales and sandstones 

 with mostly irregular coal seams, has at base a well-defined per- 

 sistent coal, known as Mynyddislwyn, Llanwit I, Wernfifraith and 

 Swansea in different districts. A vertical section of iii feet in 

 the Blackwood Valley near Newport of Monmouthshire, shows six 

 coals, 6 to 30 inches thick, but they are extremely irregular. The 

 Mynyddislwyn is double and the parting varies in thickness ; in one 

 area, it varies from 2 to 24 feet; in another, from 6 inches to 15 

 feet, while in another the parting of 2 feet becomes 15 feet within a 

 short distance. Crossbedded sandstone is not unusual in the eastern 

 part of the field. 



The middle division or Pennant Grit is for the most part a clayey 

 somewhat feldspathic rock at the east, which thickens very rapidly 

 toward the west, where it is broken by shales. No workable coals 

 are known at the east but southwardly and westwardly several 

 seams become workable, though as a rule the coal is of inferior 

 quality. The Tillery Coal, known in portions of the field as the 

 Brither, Rhondda 2, Ynysarwed and Garn Swilt, is at the base. 

 This Grit has occasional bands of conglomerate, containing quartz 

 pebbles and rounded fragments of ironstone, coal and Coal Meas- 

 ures rocks. In Glamorganshire, where shales are in upper part of 

 the Pennant, there are three workable seams, while farther west 

 some coals are important. A crossbedded sandstone is the roof of 

 Rhondda 2 ; at others that roof is conglomerate with pebbles of iron- 

 stone and coal ; this is common in western localities. 



The Lower or Steam Coal Series consists very largely of shale 

 at the east but sandstone increases toward the west. It contains 

 the coals which have made the field so important. In the upper por- 

 tion, below the Tillery Coal, there occurs a notable thickness of red 

 shale, which is .very characteristic along the eastern side. The 

 rocks generally show much variation, thickening rapidly toward the 

 west, where they become coarser. 



The coal seams change much in structure as well as in quality, 

 but some of them are so persistent as to be definite stratigraphical 

 horizons. The best-marked seam is that known as the Rock, Black, 



