32 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



Russell asserts that the coal seams of the Richmond basin are 

 irregular and greatly disturbed by faulting. They are not continu- 

 ous though they are approximately at the same horizon. He regards 

 them as overlapping lenses, individual deposits thinning away. A 

 thin seam in one mine may be the important one in another. As to 

 the interval to the granite, he cites O. J. Heinrich, who in 1879 re- 

 ported that at Midlothian the coal is at 570 feet from the granite ; 

 also Fontaine, who in 1883 stated that the interval at Clover Hill 

 is 250 feet. Russell suggests that the luxuriant subtropical vege- 

 tation of these Triassic lowlands has its nearest modern analogue 

 in the fern forests of New Zealand. The ground must have been 

 covered with ferns, above which rose equiseta and the great ferns 

 with palm-like leaves ; cycad forests with pines of Araucarian type 

 covered the upland. 



Shaler and Woodworth report that the lower barren beds, under- 

 lying the coal measures, are not always recognizable with certainty ; 

 sometimes the barrenness may be due to lack of coal accumulation 

 at the locality, but there are places where the coal group is fully 

 developed and where a considerable thickness of barren rocks was 

 seen. These authors offer no explanation of the origin of the 

 bowlder beds occasionally observed at the base of the section. They 

 consist of granitic bowlders with a partial bedding of reddish gritty 

 sandstone. Plate XXI. of the report illustrates well the disin- 

 tegration of the granite, which proceded deposition of Trias in this 

 basin. This is remarkably similar to conditions observed by the 

 writer in central France between Aurillac and Decazeville, where 

 such disintegration is shown at many places. In the Decazeville 

 basin, this preceded the deposition of the Coal Measures and the 

 accumulations were mistaken for deltas by several observers. 



Some have supposed that the great variations in thickness of 

 the Richmond seams were caused by pressure during disturbance ; 

 but there appears to be no reason for resort to this explanation. 

 Such swelling and contraction of seams is certainly common enough 

 in disturbed regions, but there the structure of the coal is changed; 

 it is exceedingly tender or it is rolled into flakes like pastry. But 

 in the Richmond basin the lamination, according to Taylor and 

 according to observations by the writer, is undisturbed in locali- 



