524 Noble — Geology of the Grand Canyon, Arizona. 



found covering the plain, and of the climate of the time is 

 preserved for us in the basal conglomerate described in a 

 previous part of this article. It was there noted that the 

 weathering below the surface of the peneplain was slight, and 

 the product of physical disintegration rather than of chemical 

 weathering ; that the matrix of the conglomerate is red in color 

 and arkose in nature ; that the pebbles are angular and show 

 no evidence of transportation and sorting. We may therefore 

 conclude that the conglomerate represents the soil in place 

 which covered the Yishnu plain. All the evidence points 

 to an arid climate, — the lack of chemical weathering, the 

 freshness of the arkose feldspar, and the red color of the matrix, 

 all indicating a lack of vegetation and abundant moisture which 

 could decompose the soil and reduce the iron which imparts 

 the red color. It is therefore not unlikely that the Yishnu 

 plain was a vast desert at the incoming of the sea. The abso- 

 lute lack of transportation, sorting, and rounding of pebbles 

 indicates that the incoming sea had little chance to rework 

 the soil mantle by its waves. It seems impossible to account 

 for this phenomenon except by a sudden invasion of the sea 

 across the Yishnu desert. If we interpret the past in the light 

 of the present our only guide is to seek to picture some present 

 condition on the earth which parallels that preserved in the 

 geological record. A possible clue may lie in the conditions 

 about the Caspian Sea to-day. There is in that region a desert 

 about the shores of the Caspian which lies below the present 

 level of the Black Sea ; a sudden rise in the level of the ocean 

 might cause the latter to overflow the low barrier which sepa- 

 rates it from the Caspian, and in this way a sudden inundation 

 of the desert would be accomplished. It is now thought that 

 wind erosion may carry the surface of an old desert to a level 

 below that of the sea if the cycle is continued to extreme 

 maturity. Such forces might have been active in the last 

 stages of the Yishnu peneplain. 



The deposition of the alternating limestones and shales of 

 the second member of the Unkar seems to have taken place in 

 a permanent water body into which mud was frequently 

 washed. It may be noticed from the section of this member 

 that the alternations are almost innumerable. The exact cause 

 of this is speculative. Possibly the alternations are due to 

 climatic oscillation : a movement from an arid to a semi-arid 

 climate would load the rivers with sediment, while arid 

 intervals would retard their flow, if not dry them up entirely, 

 resulting in a temporary clarifying of the sea and a deposition 

 of limestone. Whether the limestones are the result of organic 

 agencies or of purely chemical precipitation is also a matter of 

 speculation, since in regard to decisive evidences of life the 



