GEOLOGICAL SOCIETY OF PENNSYLVANIA. 



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We have not observed any of those bodies or fishes so 

 abundant in the coal slates, according to Mr Nuttall, in 

 this district. It is not improbable but their forms have 

 been mistaken for the detached fronds of neuropteris or 

 pecopteris. 



It is not the least remarkable geological feature of this 

 district, that throughout such repeated alternations of 

 rock beds, gritty argillaceous and coal shales, one distin- 

 guishing character in the structure of the compact rocks 

 may be traced through the entire series. That a great 

 accumulation of rocky beds, of mechanical origin, clearly 

 and solely derived from primitive rocks, should be lodged 

 within a basin of granite, is by no means difficult to ac- 

 count for. But that rocks possessing this conglomerate 

 character should be interstratified with innumerable 

 argillaceous and carboniferous beds, which attest long 

 and undisturbed intervals between each alternation, in- 

 tervals at any rate long enough to admit of the growth 

 of plants of considerable magnitude, suggests a remarka- 

 ble duration or extension of that epoch during which 

 those repeated supplies of similar constituents, derived 

 apparently from a common source, continued to proceed. 



What may be the greatest depth of this coal basin 

 must, in the absence of evidence, remain purely conjec- 

 tural. Like most deposits of this description, it is pro- 

 bable that the central beds are flattened, and for a certain 

 space towards the centre are horizontal. We perceive, 

 for instance, on the west side of the basin, that the decli- 

 vity of the lowest beds, which was at first as great as 

 thirty degrees, decreases even in the workings to fifteen 

 degrees; and that at the distance of two or three miles 

 this angle, judging from the dip of the upper beds which 

 alone can be examined, is diminished to two or three 

 degrees only from horizontality. Something like this is 

 noticed in the mines on the eastern side of the basin, 

 where the descent is stated to commence at forty-live 



