456 J. BAREELL OBIGIN OF THE MAUCH CHUNK SHALE 



other hand, while the strata have been tilted beyond the vertical, the dip 

 is constant throughout the section, and the depth of the bottom of the 

 syncline at that place prevents the present exposed portion from being 

 near the region of changing dip. Observation does not indicate any 

 lateral adjustment between beds. 



Inorganic evidences of subaerial exposure. — These shaly partings, as 

 noted by H. D. Eogers, frequently show a glazed surface, which he con- 

 siders as an indication of the freshly deposited clay having been exposed 

 to the air in a wet state. He further remarks : 



"These glazed surfaces not only sometimes retain the impressions of delicate 

 water marks and groovings such as water trickling down a slimy or wet sandy 

 beach always produces, but are sometimes imprinted with the markings called 

 'rain spots,' and more rarely the footprints of land animals or cracks filled with 

 sand, such as geologists are wont to attribute to shrinkage in mud from the 

 sun's heat."* 



The best exposures among those examined by the writer were at Potts- 

 ville, on the west side of the river. The lower 400 feet (estimated) do not 

 show rock outcrops, but from the development of the drainage and the 

 character of the soil the strata are judged to form predominantly, if not 

 entirely, a shaly mass. The middle two-thirds of the formation are more 

 arenaceous and resistant and the vertical strata cut across by the river 

 furnish ideal opportunities for studying the sequence of the beds. The 

 exposures have been increased in recent years by excavations into the 

 cliffs to allow laborers' cottages to be built between them and the road- 

 way, and on account of the rapid weathering of the shale surfaces they 

 are doubtless now at their best. 



Beginning with the first good exposures, not more than 500 to 550 

 feet from the top of the Pocono, the first unmistakable mud-cracked 

 surfaces appear on the incoherent, tliin and lustrous, shaly laminae sep- 

 arating the sandstone beds. Tn close.contiguity with this lowest observed 

 mud-cracked stratum is one of calcareous shale holding lime concretions 

 2 inches in diameter. The view in plate 49 is taken a little above this 

 horizon. From this point upward through the next 1,500 feet of strata 

 strikingly mud-cracked surfaces are found at short intervals, a view in 

 the upper portion being shown in plate 50, while on many intermediate 

 beds more or less faint patterns of mud-crack polj^gons may be seen by 

 careful observation. Certain of these would not be positively identified as 

 mud-cracked surfaces were it not for the association with adjacent strata 

 in which they are beyond doubt. In the better instances smooth shale 

 strata are intersected by polygonal ribs of sandstone from sand carried 



• Geology of Pennsylvania, vol. 11, part 11, 1858, p. 831. 



