GEOLOGICAL SOCIETY OF PENNSYLVANIA. 279 



The western outcrop is commonly termed the Upper 

 seam," and the eastern the "Lower seam/' under the 

 supposition, in connection with some observed variations 

 in quality and thickness, that they are distinct masses or 

 beds of coal. But on careful examination of the included 

 strata, the *^ measures" or " metals" as they are termed 

 in mining phraseology, and from numerous other obvious 

 characters, the geological continuity or conformity of the 

 whole becomes too apparent to leave room to doubt that 

 it is one coextensive deposit, filling up a trough of un- 

 certain depth in the primitive rock. 



On the causes or origin of this and similar basins in 

 various parts of the world, it forms no part of the plan 

 of this memoir to animadvert. 



So rapid is the descent of this rock and consequently 

 of the superincumbent coal beds, that the numerous 

 collieries within its limits, instead of being distributed, 

 form a chain of works ranging in a line along the margin, 

 both at the eastern and western outcrops, seldom ex- 

 tending further into the basin than from a fourth to a 

 third of a mile. 



The Richmond coal field ranges about N.N.E. ; but 

 its precise limits at either extremity do not appear to be 

 distinctly defined. It is understood that the extreme 

 points where coal traces exist are about thirty-five miles 

 apart ; that is to say from the Appomattox river to the 

 forks of the Pomunky. 



Our line of section is so arranged as to intersect this 

 basin at points where its internal structure is best defined 

 in the numerous shafts. The greatest breadth is here 

 ten miles. It is probable that in the direction of this 

 transverse line is comprised the greatest thickness of 

 coal, which thins oW as it approaches the northern and 

 southern extremities 5 the maximum thickness being, so 

 far as can be proved, about the centre of the eastern side. 



By comparing fig. 1 and 2, pi. 16, which represent 



