28 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



distinct groups in the Virginia areas : the upper group, consisting of 

 loose granitic sandstone or sandy shale, containing no coal but much 

 lignite, resembling jet; silicified wood is not rare; a middle group, 

 coal-bearing, with a large proportion of black shale ; a lower group, 

 sandstone and shale. The sandstones of the lower group are not 

 easily distinguished from the underlying granitoid gneiss and are 

 100 to 600 feet thick in the Richmond area. The middle group is 

 100 to 200 feet thick in the same field, where it usually has two 

 thick seams of coal — but the number, thickness and quality vary 

 greatly. At many places the roof is a plant-bearing shale ; Equi- 

 setuni rogersi is usually associated with Macrotccniopteris and its 

 casts are present in the coal. Schisoneura occurs in the underclay 

 of the main seam. The plants described by Fontaine are conifers, 

 cycads, equiseta and ferns. 



Shaler and Woodworth*^ applied names to Fontaine's groups ; 

 the Chesterfield or upper group is 2,500 feet thick and consists of 

 sandstone above, shales below ; the Tuckahoe, equivalent to the 

 middle and lower groups, consists of the coal measures, 500 feet, 

 more or less, sandstones and shales, o to 300 feet, and bowlders, o to 

 50 feet. 



The Richmond field was discussed many years ago by geologists, 

 who studied it when the mines were still in operation.*^ It is well 

 to summarize the statements of each observer as the conclusions 

 reached by them have been regarded as not in agreement and they 

 appear to be in some respects contrary to those reached by observers 

 who have studied the region since mining operations, practically 

 ceased. 



Taylor reported that the deposits occupy a narrow trough, which 

 deepens so rapidly toward the median line that coal mines are pos- 



'^- N. S. Shaler and J. B. Woodworth, U. S. Geol. Survey, 19th Ann. Rep., 

 Part II., 1899, p. 423. 



43 R. C. Taylor, " Memoir of a Section Passing through the Bituminous 

 Coal-Field near Richmond, Virginia," Trans. Geol. Soc. Pcnn., Part I., 1835, 

 pp. 275-297; W. B. Rogers, "Reprint of Annual Reports on Geology of Vir- 

 ginia," 1884, pp. 62-69; "On the Age of the Coal Rocks of Eastern Virginia," 

 Reps. Amer. Asso. Geol. and Nat., 1843, pp. 298-316; C. Lyell, "A Second 

 Visit to the United States of North America," 2d ed., London, 1850, pp. 

 281-287. 



