146 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



of marine limestone serves as roof to one bench and as floor to the 

 other. These Hmestones are thin but they are proof of submerg- 

 ence, due perhaps to change in course of drainage or to the breaking 

 away of a barrier, which protected the swamp from sea-invasion, a 

 by no means rare phenomenon on the New England coast. The 

 roof is apt to be irregular. 



7. The Coal Seams. — Where succession is undisturbed and dejx)- 

 sition appears to have been continuous, the roof material ordinarily 

 becomes more, and more carbonaceous at the base and passes gradu- 

 ally into bone or into impure coal, with normal structure, a faux- 

 toit. But the transition is abrupt in many cases where no evidence 

 of disturbance by erosion is apparent; a condition which leads to 

 the suggestion that a suddenly increased influx of mud or fine sand 

 ended the bog's existence. In such cases the contact between coal 

 and roof is irregular, defining the bog surface. 



Accumulation of vegetable material was rarely continuous during 

 long periods, though there are seams several feet thick, which are 

 said to be unbroken by partings of any sort. Commonly, however, 

 coal seams are divided into benches by partings of mineral charcoal, 

 clay, sand or limestone, which indicate longer or shorter periods of 

 interruption. In many cases, this interruption was not complete and 

 the parting consists of bone or bony coal, at times closely resembling 

 cannel ; but when the parting consists of inorganic matter, it is 

 proof of at least local cessation. The thickness of partings usually 

 varies within narrow limits, but in some cases it is so great as to 

 attract the attention of even a casual observer. Czjzek notes the 

 thinning away of a considerable interval and the consequent union 

 of two important seams, with increased thickness of coal. In the 

 Denver Basin, one parting increases from a mere film to 25 feet 

 within a few miles ; the partings in the Carl:)onero seam of the San 

 Juan Basin thicken in one direction, so that the great bed, 100 feet 

 thick, becomes three, with thicknesses of 7, 30 and 15 feet respec- 

 tively, in a vertical space of 200 feet. Tafif describes a parting, which 

 increases from zero to 16 feet within 2,000 feet, the exposures being 

 complete in one mine. The Trinidad seam, 11 feet thick near 

 Trinidad, Colorado, becomes 58 feet within 3 miles by thickening 

 of the partings. Lee has given details making almost certain that 



