GEOLOGICAL IMPORTANCE OF SEDIMENTATION 559 



and attrition characteristic of the work of currents and waves or 

 of desert winds resulting in the accumulation of dune sands. The 

 latter idea is perhaps made improbable by the transition into, and 

 alternation with, the deoxidized and carbonaceous lower members 

 of the Chamberlain shales. 



The Chamberlain shales grade, on the one hand, into the under- 

 lying quartzite and, on the other, into the Newland limestone. These 

 relations, in addition to the dark silicious and occasionally arena- 

 ceous character and occasional ripple-marks, suggest a quiet off- 

 shore formation. The mud-cracks noted by Walcott may be either 

 of Mttoral or fluvial origin, but in either case imply a nearby land. 

 The lack of a more arenaceous character may be due, therefore, 

 to a topographic old age and lessened stream gradients of the land, 

 or to the river material having been borne from a great distance. 



Newland and Altyn limestones. — The inauguration of the era 

 of the Newland and Altyn limestones may be due as much to the 

 lack of supply of mechanical sediments as to subsidence and incur- 

 sion of the sea. 



Grey son and Appekunny argillites. — Following the limestone came 

 some 2,000 to 3,000 feet of Grey son and Appekunny argillites. 

 The generally dark gray color, thin-bedded lamination, occasional 

 ripple-marks, and quartzitic strata suggest the submarine deposits 

 poured into a sea as a result of the re- elevation of a contiguous land. 



The association with the limestone below, the absence of con- 

 glomerates and observed mud-cracks, and the contrast in color 

 with the deep-red and mud-cracked formation above all tend to 

 confirm this interpretation. 



The Spokane and Grinnell argillites^ from 1,000 to 2,000 feet 

 in thickness, on this view represent subaerial delta deposits over a 

 region where sedimentation had gained upon subsidence to such an 

 extent as to fill up and exclude the sea. In contrast to the inferior 

 argillites are to be noted the highly oxidized character indicated 

 by the deep red color, the frequent alternations of sandstone strata, 

 and especially the widespread occurrence of mud-cracks. These 

 are not sparingly present and developed in strata transitional between 

 two distinct types, as would be characteristic of mud-cracks of the 

 littoral zone, but on the contrary are developed in the normal red 

 shales. Furthermore, the exposed sections show throughout a 



