506 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



cession as determined by him is : Coal Measure, i,ooo feet, red sand- 

 stones above, coal seams in lower portion ; Millstone Grit, 500 feet, 

 without coal seams ; Carboniferous Limestone, 2,000 feet, limestone, 

 volcanic bed, coal seams, the Hurlet limestone at base; Calciferous 

 Sandstone, divided into (i) Oil Shale Group, 4,000 feet, with 2 

 thin coals in upper part and oil shales in middle and lower parts; 

 (2) Cement Stone group, without coal and resting on the Old Red 

 Sandstone. 



The great Carboniferous Limestone, thousands of feet thick in 

 portions of England, is split up here into not more than a half dozen 

 beds, each at most 50 feet thick, with sandstones, shales and coal 

 seams in the intervals. There are many coals, almost 50, and at 

 least 17 of them exceed 2 feet in thickness. One has maximum of 

 8 feet and another of almost 6 feet. These are thoroughly typical 

 and rest on underclays with abundant Stigmaria in place. Iron- 

 stone, economically important, occurs at many horizons. At Bridge- 

 ness in the Bo'ness area, Cadell more than once explored an old 

 forest exposed by workings on the Craw Coal. On one occasion, 

 he counted 113 stumps, Sigillaria, distributed along 400 yards of 

 roadway. They were arranged in clumps and were from two and 

 one half inches to two and one half feet in diameter. The stems in 

 great proportion were prostrate. Cadell conceives that they were 

 broken off by a violent wind from the south, as most of them lie 

 over toward the north. The vertical stumps were filled with fer- 

 ruginous mud and the bark remained as coal. One of the sand- 

 stones is ripple-marked, has casts of fresh-water shells and flattened 

 heaps of worm-castings. 



Two thin coals, Two-feet and Houston, are about 1,000 feet 

 below the Hurlet limestone ; they are true coal seams but are very 

 high in ash. The Houston, at one place, is 5 feet 9 inches in 4 

 benches, including a 2-inch cannel, directly under the top bench ; at 

 another, it is somewhat more than 11 feet and has a bench of oil 

 shale. The coal is soft, but at its best is pyritous and dirty. 



The notable feature of the group is the Oil Shales, which is 

 easily recognized. It gives a brown streak, is tough, resists the 

 weather and is not gritty. The thickness varies ; at times a deposit 

 disappears or passes into ordinary shale; at others, it may reach 15 



