458 J. BARRELL ORIGIN OF THE MAUCH CHUNK SHALE 



indistinct or indefinite the rock is -usually a thick and fissile, sandy, red 

 shale, the cracks having been filled with material of the same nature. 

 The talus from such an exposure consists of fragments smaller than the 

 polygons, and hence the mud-cracks can seldom be observed upon them. 

 Upon a sufficiently broad exposure of the bedding plane, however, nearly 

 tangent sunlight may bring out a shadowy pattern standing in slight 

 relief. Upoii the well cracked surfaces the cracks average from 0.5 to 

 1.0 inch in width, the irregular polygons from 0.5 to 2.0 feet in diameter. 

 The depth of the crack appears to be limited by the depth of the stratum, 

 and is never traceable for more than 0.5 to 1.0 inch. Dimpled surfaces 

 were occasionally observed, but the writer is not certain that the ones 

 seen at this place were rain-prints. Eogers, however, observing other 

 surfaces, reports rain-prints from this section. The definitely mud- 

 cracked strata cease, on ascending through the formation, opposite the 

 wagon bridge across the Schuylkill probably about three-fourths of the 

 height from the bottom of the formation, and the rock becomes more of 

 a sandy shale. Mud-cracked patterns are thought to be shadowed forth, 

 however, in one or two places during the next couple of hundred feet. 

 Here a lateral valley intervenes, and to the north of it occur the upper- 

 most Mauch Chunk and the transition beds to the Pottsville. The out- 

 crop of these transition beds, consisting of red shale, light colored sand- 

 stone, and conglomerate, while, showing the sequence of the beds and 

 giving good exposures of the conglomerate, do not exhibit the bedding 

 planes of the shales. 



It is thus to be concluded that at Pottsville, where the formation 

 reaches its greatest thickness, at least two-thirds of it, including all the 

 central portion, were formed under conditions which permitted frequent 

 drying in the air of the successive strata previous to their burial. 



Near the village of Dauphin, on the Susquehanna, 45 miles southwest 

 of Pottsville, on the strike of the formation, H. D. Eogers noted the 

 presence of mud-cracks exposed in a roadside quarry, and accompanies 

 his description with an illustration. He states in 1858 that "frost and 

 weather have long since defaced the surface, which, when it was freshly 

 uncovered, many years ago, offered a striking example of the phenome- 

 non."* This sentence of Eogers emphasizes an important point in con- 

 nection with evidences of continental origin of argillaceous strata and one 

 already partly stated earlier in this article — that in shale rocks such 

 features as mud-cracks, especially where filling and matrix are alike, 

 can usually only be well observed upon fresh exposures of the bedding 

 planes. It is possible to study an entire section of natural outcrops of 



• Geology of Pennsylvania, vol. il, part ii, p. 831. 



