STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 499 



not far from 2,000 feet thick, consists of reddish shales and sand- 

 stones with some thin Hmestones. At Bradford coUiery, near Ard- 

 wick, 7 coal seams, 10 inches to 3 feet 6 inches, were found in a 

 section of about 700 feet. These have been exhausted. The Mid- 

 dle Coal Measures, not far from 3,000 feet thick, contains about 10 

 workable seams, which are practically persistent, though some of 

 them vary greatly. The coal is apt to be inferior when the thick- 

 ness exceeds 4 feet, as it is injured by increasing number of thin 

 dirt-bands. The Wigan cannel has abundance of fish remains and 

 Stigmaria. The sedimentary deposits are extremely irregular, hun- 

 dreds of feet of shale at one place being represented by a few feet 

 of sandstone at another. A notable mass of red sandstone, with 

 plant remains and 146 feet thick, rests on the Blenfire Coal at Glod- 

 wick colliery in the extreme eastern part of the field. The Lower 

 Coal Measures, about 1,200 feet thick, has numerous seams but, for 

 the most part, they are thin. The Bassey or Salts Mine Coal has a 

 maximum of 23 feet, but its coal is inferior and little used. The 

 Canister, where thickest, has two benches. Upper Foot and Canister ; 

 when united, the bed has thickness of 8 feet, but in a large area 

 these are separated, the interval reaching 30 feet, and the benches 

 become 2 feet 6 inches and 8 inches. The Millstone Crit, about 

 5,000 feet thick, has a thin coal seam in the upper division or Rough 

 Rock, and another lower down. Casts of Lepidodendron, Sigil- 

 laria and Calamites are numerous in several sandstones and the 

 shales often yield marine fossils. 



HuU's^"^ studies have supplied most of the information available 

 for this field. In one of his memoirs, he has described in detail the 

 Wigan area, central in the field. The Permian, chiefly red sand- 

 stone, is not found anywhere in contact with the Coal Measures, but 

 the unconformity is beyond doubt, as Upper Coal Measures are not 

 present at some localities where undoubted Permian occurs. It 

 contains no coal. 



The Upper Coal Measures, about 1,500 feet thick, red and gray 

 sandstones and marls with bands of limestone, has no workable 

 coals. The Middle Coal Measures, about 2,500 feet thick, and con- 



101 E. Hull, " The Geology of the Country Around Wigan," Mem. Geol. 

 Survey, 2d Ed., 1862, pp. 1-39. 



