MAUCH CHUNK OF THE ANTHKACITE COAL BASINS 



457 



into the cracks during the deposition of the overlying sand beds. Usually 

 most of the shale has scaled away, leaving the polygonal sandy frame- 

 work standing slightly in relief. 



Where the surfaces of the mud-cracked shale laminse are well preserved 

 they are frequently seen to be plate-like polygons with raised rims, giving 

 surfaces concave upward. Such warped as well as mud-cracked strata 

 are commonly observed on modern dried mud-flats, the structure result- 

 ing from the drying, shrinking, and hardening of the upper side while 

 the lower is still moist. It is seldom that the relations of the shale 

 stratum to the sandstones both above and below are well exposed, either 

 the upper or lower stratum being broken away. In one instance, how- 

 ever, in the middle of the Mauch Chunk formation, where both sandstone 

 layers were preserved, the overlying sandstone passed without interrup- 

 tion down into the cracks and spread out below the shale stratum, filling 

 the space made b}^ the upturned rims of the polygonal disks. These 

 underspaces were about half an inch wide at the middle and tapered to 

 a knife-edge at about 3 to 4 inches each side of the crack, as in the 

 accompanying sketch. 



Scale in inches 



Figure 1. — Detail of Miid-craclcs in Mauch Chunk Shale. 



View taken looking at the edge of a mud-cracked stratum from the locality shown In 



plate 50. 



From experiments made by the writer, it does not seem probable that 

 such mud-cracked clay layers could hold up the niargins of their disks 

 upon being rewet until the sweep of the waters had filled the underspaces 

 with a clean sand, especially when, as in this case, the mud-cracked 

 strata were of a loamy nature. On the other hand, the structure cor- 

 responds to what may be observed on the floodplains of semiarid regions, 

 where during the dry season the channel sands are swept by the winds 

 across the mud-cracked flats, drifting them full of sand and thus pre- 

 serving them before they are rewet. At places where the cracks are 



