462 J. BARRELL ORIGIN OF THE MAUCH CHUNK SHALE 



First. The hollow, Jointed, slender, reed}^ fragments suggest the equi- 

 setse. The siliceous nature of their tissues is well known, and these are 

 the only plant impressions in which any of the original substance remains, 

 though the carbon has completely disappeared. 



Second. The impressions of the flattened leaves suggest affinities with 

 the cordaites. 



Third. The spiny leafage resembles the lycopodiaceous plants. In 

 none of these could Professor Evans see resemblances to the algge with 

 which he was familiar. 



Fourth. The root markings suggest a plant which drew nourisment 

 from the soil. The larger marine algae, on the contrary, customarily are 

 attached to stones hj means of hold-fasts. The smaller marine algse 

 sometimes develop root-like hold-fasts where growing on muddy or 

 sandy bottoms, but these in modern forms are always small and delicate. 



Plant remains, for the purpose of the present paper, are not of high 

 value unless there is evidence of burial in the place of growth, as in the 

 case of the root impressions, but it is seen that, in as far as they go, 

 they are suggestive of a land origin. The manner of preserval of the 

 remains is, however, most suggestive. No carbon is in any case pre- 

 served ; neither has the loss of carbon decolorized the contiguous shale, 

 as is often the case in carbonaceous strata. On the contrary, the plant 

 impressions are simply smooth lustrous patterns, marked out in the 

 duller background of red shale, but of precisely the same color value. 

 This implies complete oxidation of the carbon by free oxygen, and not 

 by oxygen from the ferric oxide. It occurred, therefore, before the deep 

 burial of the accumulating strata, but after the superficial burial of the 

 plant fragments. 



Part II. Inferred Conditions of Origin 



PREVIOUS OPINIONS 



Eogers ascribed to the Mauch Chunk an origin in a shallow sea, 



"contiguous to agitated. coasts swept by turbid currents, a sea too foul with 

 poisonous sediments to permit the presence of the usual marine animals ; and 

 the abundance of ripple-marks, sun-cracks, and the specks attributed to rain, 

 and called rain-spots, confirm the impression of the nearness of the land by 

 giving proof that the layers, while yet freshly deposited, were frequently laid 

 naked to the atmosphere."* 



Eogers is thus seen to have framed an explanation which reconciled 

 the universal opinion of his time, that sedimentation was essentially a 

 marine or lacustrine phenomenon, with his own observations indicating 



* Geology of Pennsylvania, vol. ii, part ii, 1858, p. 794. 



