450 J. BARBELL ORIGIN OF THE MAUCH CHUNK SHALE 



The first part of tlie present paper consists of a presentation of the 

 significant facts of the Mauch Chunk shales, gathered both from the 

 literature and from personal observation. The second part deals with 

 the interpretation of these facts. Where a new view is to be proved the 

 observational and inferential portions should be strictly separated, but 

 at the same time the purpose for which the facts are presented should be 

 evident when they are given. It is therefore desirable to state in advance 

 the conclusion finally reached, which is that in the anthracite region, 

 more surely in the southeastern and eastern portions, the whole forma- 

 tion, from top to bottom, was a subaerial delta deposit laid down under a 

 semiarid climate. 



Besides merely the essential facts, there are included in the descriptive 

 portion many details which are not used in drawing the conclusions, but 

 which, if the hypothesis of terrestrial deposition be accepted, are interest- 

 ing as showing in a definite case what the minor characteristics of such a 

 formation may be. They are doubtless also valuable as indicating specific 

 features of climate or geography, even though their significance may not 

 at present be understood. 



GENERAL DESCRIPTION OF THE MAUCH CHUNK FORMATION 



The Mauch Chunk shale is the name of a strongly individualized for- 

 mation of the Lower Carboniferous of Pennsylvania, chiefly consisting, 

 in the region of its typical development, of alternating red shales and red 

 sandstones, the ratio of the two varying through difl^erent horizons of the 

 formation and in different districts. 



Not a little of the distinctive character of the Mauch Chunk is due to 

 its sharp contrast with the preceding and succeeding formations. The 

 inferior, the Pocono sandstone, consists of a great thickness of grayish 

 green to light gray, cross-bedded sandstones, the whole weathering white 

 and constituting a massive and. resistant formation, topographically 

 prominent wherever it occurs. Above the Mauch Chunk lies the Potts- 

 ville conglomerate, interstratified with gray sandstones. The Pottsville, 

 like the Pocono, is a resistant formation and one to whose protecting 

 rim many of the smaller synclinal coal basins owe the preservation of 

 their coal during the Tertiary cycle of erosion. In contrast with these 

 adjacent formations the Mauch Chunk as a whole is weak and conse- 

 quently readily reduced to local base level. It yields a red loamy soil, 

 and where the formation is thickest, as around the southern and middle 

 anthracite coal basins, gives rise to comparatively level and broadly open 

 valleys. These are utilized for agriculture, and the checker-work of 

 fields contrasts strikingly with the forest-covered and level-topped slopes 



