CLIMATE AND TERRESTRIAL DEPOSITS 283 



Goldthwait have raised the question if such has not been the mode of 

 origin of the Triassic and Jurassic sedimentary formations of the 

 Colorado plateaus.^ 



Third, scattered and facetted pebbles. — In truly aeolian sands the 

 size of the material is rather sharply limited, but in stream channels 

 pebbles or larger fragments may be carried indefinite distances from 

 the fields of erosion. Cloud-bursts and sheet-flood deposition may 

 also sweep large fragments along with the fine, but such action is 

 necessarily more closely limited to the vicinity of the sources of sedi- 

 mentary supply. In such ways pebbles of various sizes may be clus- 

 tered or scattered through a finer textured deposit without necessarily 

 implying transportation by either the roots of floating trees, or floating, 

 or glacial ice. Pebbles carried by each of these agencies will be apt 

 to exhibit distinctive characteristics and associations. Those carried 

 by fluvial or pluvial action upon arid flood-plains are the ones to be 

 here considered. The association with various indications of climatic 

 conditions such as those already considered may often suggest the 

 mode of origin. It seems probable, however, that in many instances 

 such pebbles should carry their own evidence through being sub- 

 jected to wind scour upon the drifting away of the enveloping sand. 

 They would become facetted in consequence, giving rise in the con- 

 solidated conglomeratic sandstone to the occasional presence of 

 "dreikanter." Such pecuharly facetted but unstriated pebbles have 

 been collected from a number of formations, dating back even to the 

 pre-Cambrian, but have sometimes been claimed to be of glacial 

 origin. An instance where the hypothesis of aeolian origin appears 

 to offer by far the best explanations has recently been described with 

 illustrations by Lisboa in the case of pebbles which are probably of 

 upper Mesozoic age from the central plateau of Brazil.^ 



Organic characteristics. — Over the more truly desert portions of 

 arid flood plains the life is unimportant at the present time, and in 

 past times has been equally unimportant, if not more so, since the 



1 E. Huntington and J. W. Goldthwait, "The Hurricane Fault in the Toqueville 

 District, Utah," Bulletin of the Museum 0} Comparative Zoology at Harvard College, 

 Geological Series, Vol. VIj No. 5, 1904, pp. 210-17. 



2 "The Occurrence of Facetted Pebbles on the Central Plateau of Brazil," Ameri- 

 can Journal oj Science, Vol. XXIII, 1907. pp. 9-19. 



