470 J. BARRELL ORIGIN OF THE MAUCH CHUNK SHALE 



grades were flattest and swamp conditions most favored^ yet such deposits 

 only occur in the upper shales. These relations suggest that the later 

 part of the Mauch Chunk epoch was marked by a series of climatic oscil- 

 lations toward the conditions prevailing in Pottsville times — conditions 

 which there are reasons shown elsewhere for believing were characterized 

 by a heavier rainfall and probably cooler climate. On the other hand, 

 gypsum and salt are not known to be present in noticeable amounts in any 

 part of the Mauch Chunk, although much gypsum and considerable salt 

 are present in the shales and argillaceous limestones of the Michigan 

 series of Michigan, which is probably to be correlated with the Lower 

 Mauch Chunk. Thus the indications point toward the climate of this 

 epoch in Pennsylvania being of a semiarid character, seasonal rains being 

 sufficient in amount, in conjunction with the discharge of large rivers, to 

 prevent the precipitation of salt or gypsum on the marginal fl^ats facing 

 the Appalachian gulf of the Mississippian sea, but at times permitting 

 such in the more isolated Michigan basin. 



The semiaridity of a climate with a slight or a seasonal rainfall is 

 accentuated by high temperature. That the climates of sub-Carbonifer- 

 ous times were marked at least in part by warmth is indicated by the 

 fossil plants and corals of so northerly a land as Spitzbergen. 



The semiaridity of the Mauch Chunk climate of Pennsylvania is fur- 

 ther indicated by a number of chemical and textural features characteris- 

 tic of the formation in the region of the anthracite basins. Here an 

 appreciable amount of feldspar grains intermingled with the quartz grains 

 of the sandstones are detected by the microscope and calcite is fairly 

 abundant in the matrix. This abundance is expressed megascopically on 

 many horizons by the development of concretionary nodules usually in 

 certain stratigraphic planes, the close association with mud-cracked strata 

 in at least one instance indicating that they are not, however, the calcare- 

 ous deposits transitional to an open sea. In the fine-grained and shaly 

 sandstones an examination by the microscope shows that both muscovite 

 and calcite are conspicuously present. Hematite, as previously stated, is 

 diffused through both shales and sandstones, though naturally more 

 abundant in the former. 



The interpretation of these facts would appear to be as follows : 



The feldspar present after prolonged transportation indicates a notable 

 degree of ph3rsical as contrasted to chemical Aveathering in the regions of 

 erosion. The presence of the imiformly diffused iron oxide points to a 

 lack of leaching by organic acids either at the headwaters or upon the 

 drained portions of the floodplain surface. The presence of the calcite 

 in slight amount is characteristic of all floodplains, but is only found to 



