MAUCH CHUNK OF THE ANTHRACITE COAL BASINS 455 



forms a greater ratio of the whole at Solomons gap, where the formation 

 is thinner. 



The sandstone members without notable shale are seldom thick, in 

 only one case exceeding 31 feet, and, as observed by the writer, are hard 

 and fine grained. The individual strata possess an average thickness of 

 perhaps a foot, but often of not more than an inch. In the shale sand- 

 stone portions the individual beds of sandstone are separated from each 

 other by thin partings of shale. The sandstone strata are continuous 

 over the limits of any one exposure, and no marked bottom channels are 

 to be observed where the sandstone rests on shale. All gradations from 

 hard, somewhat ferruginous sandstones through argillaceous sandstones 

 to sandy shales are abundant. Eipple marks, wave marks, and current 

 marks are common upon the surfaces of the arenaceous beds. Cross- 

 bedding is common, and a small amount of calcium carbonate is fre- 

 quently present, as shown by calcite infiltrations into fracture planes 

 and as seen under the microscope. 



The argillaceous strata where forming thick masses are very seldom 

 well exposed, the weathering of a few years sufficing to cause recently 

 excavated material to crumble into a soft rubbish, as may be observed 

 in the railroad cuttings and fillings. The surfaces of shale where re- 

 maining exposed develop such a close and irregular fissility that observa- 

 tions on the original nature of the bedding planes are impossible. Usu- 

 ally such massive shales possess a perceptible amount of siliceous grit, 

 and have resulted from the consolidation of loamy clays. The shale 

 layers may consequently be best studied where they form the partings 

 between the sandstone beds of the shale and sandstone members, having 

 been protected from weathering in such places by the more resistant 

 strata. Detailed examination was made by the writer, to such extent 

 as time permitted, of the section cut by the Lehigh river at Mauch Chunk, 

 and more thoroughly of the section along the Schuylkill river at Potts- 

 ville. The former showed very little structure in the shale beds, while 

 at Pottsville the original structures were excellently preserved. The 

 difference between these two regions is to be ascribed with high probability 

 to the' fact that the section at Mauch Chunk cuts across the compressed 

 end of the coal basin syncline. The consequent changes in dip within 

 short distances must have involved during the folding considerable ad- 

 justment of adjacent strong beds upon each other, as is in fact testified 

 to by the numerous minor slip planes and fractures in the sandstones. 

 Under such circumstances the shaly partings,^while still protected from 

 weathering, seem to have formed planes of adjustment with more or less 

 obliteration by shear of the original features. At Pottsville, on the 



