CLIMATIC CONDITIONS 471 



the extent here present in those of semiarid climates. In fact, as previ- 

 ously noted, the calcite throughout much of the Mauch Chunk is so 

 abundant as to form concretionary bands analogous to the kankar of the 

 hot and semiarid portions of the Indo-Gangetic plains. That seasonal 

 drying and consequent aeration of the soil took place to the greatest depth 

 of root penetration is indicated by the universal elimination of organic 

 matter by oxidation without accompanying reduction of the intermingled 

 iron oxide to the ferrous state, the plant impressions being always of the 

 same color as the surrounding rock. The oxj^gen used up in the process 

 was consequently atmospheric oxygen, and the blue and gray muds of 

 swamps and river bottoms in our own climate testify to the difficulty of 

 free oxidation of organic matter in a water-saturated soil where the sur- 

 face remains moist, even though there may be no superficial standing 

 water. 



These conclusions are further supplemented by the presence of many 

 mud-cracked strata, since these are prominent features of modern flood- 

 plain deposits of argillaceous character in arid and semiarid climates. 

 They are not entirely absent from the clays of floodplains under some- 

 what humid climates, but are there much restricted in development, owing 

 to the mat of vegetation and the greater ratio of rainfall to evaporation. 

 The details of the mud-cracks as observed in the Mauch Chunk at Potts- 

 ville further emphasize the conclusion, since they seem to have been filled 

 during the dry seasons by wind-drifted sands — a phenomenon which may 

 be observed today in certain arid and semiarid regions. 



Before closing this topic of the climatic inferences it seems desirable to 

 call attention to the absence of notable bottom channeling exhibited by 

 the sandstone strata, the contacts of the sandstone with the underlying 

 shale being straight and conformable within the limits of a single ex- 

 posure. Such a characteristic at first sight seems incompatible with the 

 idea of stream channels wandering across the surface of a fioodplain. 



It is to be noted, however, that many fioodplain deposits of the western 

 United States and of other regions show in section well developed strati- 

 fication with but little marked bellying downward of the sandstone strata 

 into the shale. Such structures are occasionally noted in the coal fields, 

 where Carboniferous river channels have cut through the coal beds, but 

 those familiar with the coal mines Imow how unusual is this feature and 

 how rarely nature would expose it at the surface in normal outcrop. Yet 

 it seems certain that in many coal fields at least the greater portion of 

 the strata are of continental origin. It would appear, therefore, that 

 until the details of the sandstone strata are worked out with completeness 

 the absence of such relations visibly exposed must not weigh against a 

 hypothesis of subaerial origin. 



