The Shimtmo Area. 525 



geological record is silent. It is believed that the water 

 body was for the most part shallow, since sun-cracks appear in 

 the shales in the upper part of the member below the highest 

 limestone strata. What the polygonal cracks and the small 

 cubic depressions in the banded cherty layers of the basal lime- 

 stones mean the writer does not know. The cracks are 

 suggestive of sun-cracks and the depressions strongly resemble 

 salt hoppers. 



The most striking feature of the third member is the great 

 abundance of sun-cracks throughout the shaly strata and of 

 ripple-marks and of cross-bedding in the sandstones. The 

 researches of Professor Barrell have made it clear that exten- 

 sive sun-cracking is a feature which has a maximum chance of 

 preserval on broad flood plains or deltas in an arid climate. 

 (Barrell, a, pp. 524-568.) In the opinion of the writer of the 

 present paper, the extreme abundance of these cracks in this 

 member of the Unkar is hard to account for except by postu- 

 lating wide delta flats or flood plains. Furthermore, the bright 

 red color of the shaly strata in connection with the mud cracks 

 seems to bespeak an arid climate with little or no vegetation 

 to reduce the iron. It is at least certain that all this part of 

 the Unkar was deposited in very shallow water which often 

 evaporated entirely, leaving broad mud flats exposed to a 

 hot sun. In the upper part of the member is the series of 

 alternating shales and sandstones already described in detail : 

 as may be seen from the sections, the alternations are fre- 

 quently as regular as clockwork. The sandstone layers are 

 composed of fine, cleanly sorted and rounded quartz grains, 

 ripple-marked and cross-bedded, while the shales are a fine red 

 mud. It is thought that the clean character of the sandstone 

 layers of this alternating series is a mark of climatic oscilla- 

 tion : a climatic movement towards a wetter climate, if increas- 

 ing the ratio of run-off to erosion, causes the rivers to flow on 

 a lower grade and sweep seaward the piedmont deposits of 

 sand and gravel ; as the clay was largely sorted from those 

 deposits when they were first laid down, their redistribution 

 accompanied by a secondary sorting on a delta surface or sea- 

 bottom would be marked by extreme cleanness. 



Great thickness, clear sorting, and extreme fineness and 

 roundness of grain are the characters which distinguish the 

 fourth member, which is composed entirely of sandstones and 

 quartzites. All strata are characterized by cross-bedding and 

 ripple-marks, bespeaking shallow water. The origin of these 

 great thicknesses of sandstone is a puzzle. The clean sorting 

 seems to indicate long transportation : it is not impossible that 

 the rivers carrying this material might have flowed through a 

 great desert of dune sands, picking up and carrying material 



