8 THE OLDER MESOZOIC FLORA OF VIRGINIA. 



seem to be local. They are found confined to a space of from 100 to 200 

 feet above or below the main beds. The lower persistent bed is found near 

 the base of the middle group of the strata. South of James River it is 

 from 4 to 5 feet thick, and is worth but little for fuel in comparison with the 

 bed next above it. Hence its horizon is rarely reached. This is to be 

 regretted, as its roof-shales have many fine plants. North of James River 

 this lower bed becomes more important, apparently being there 6 to 8 

 feet thick. The interval between this and the next persistent seam above 

 is pretty constant, being from 40 to 50 feet. This second persistent seam 

 from the bottom is the main or big seam. This is always a double bed, 

 usually separated by a sandstone and shale parting. Near the James, and 

 north of it, the two members of this bed are of about equal thickness, and 

 on the north of the river this thickness diminishes. Thus at the Dover 

 Mines each member is about 6 feet thick. The interval between them here 

 reaches its maximum in some places and shows the greatest amount of 

 fluctuation in thickness. It is said to vary in no great distance from noth- 

 ing to 40 feet. Perhaps this is due in part to the great disturbance that 

 this part of the field has undergone. At Carbon Hill the interval is 17 feet 

 and under, the lower member being 4J and the upper 6 feet thick. In the 

 southern part of the field the two members of this bed attain their max- 

 imum thickness, and this they seem to do at the expense of the overlying 

 local beds. At Carbon Hill there is at least one bed 6 feet thick, 50 feet 

 above the upper member of the main seam. This does not appear to exist 

 at Midlothian. At this place Mi\ O. Heinrich gives a full account of the 

 coal beds. According to him the lower bed is 566 feet above the gneiss, 

 and is composed of 3 J feet of coal and 1 J feet of shale. Omitting a small 

 seam 12 inches thick, the next above is the main seam in two benches, the 

 lowest 44 feet above the first coal bed. Between the two benches is a 

 thickness of 10 feet of sandstone and shale. The lower bench is 12 feet 

 thick, the upper one 14^ feet. Over this come 863 feet, as far as tested, of 

 sandstones and shales, with no coal worth mentioning. 



At Clover Hill, in the southeastern end of the field, the conditions are 

 pretty much the same, except that some small coal seams occur above the 



