368 PROCEEDINGS OF THE AMERICAN ACADEMY. 



dissolved from the surface, transported in solution by the torrential 

 Btreams, and redeposited through evaporation. . . . Tepetate is forming 

 eat incrustations around the margins of the bolson plains of northern 

 Mexico" i L8th Ann. Rep., U. S. < •■ S., - 

 .Many othe aces might be made to the importance of the sub- 



ial deposits or " wash " at the ba sub-arid mountains, for Buch 



deposits are well known in Utah, Nevada, and southern California, as 

 I as in more distant parts of the world (see Hilgard, Cienegas of 

 Southern California, Bull. Geol. Soc. A.mer., iii. 127; Manual Geol. 

 ot Imiia, 2nd edition, 417, 418; Blanford, Superficial Deposits in the 

 Valleys and Deserts of Central Persia, Quart. Journ. Geol. 

 ls7:;. 193), but a return to the reports of Fortieth Parallel Survey will 

 suffice. Here one may find abundant testimony to the competence of 

 subaerial processes to form extensive deposits flanking mountain ran 

 lmt attention is given almost exclusively to the coarse, unstratified de- 

 posits that are formed by storm Hoods mar the mouths of mountain 

 valleys. For example: — "The interior valleys of the Cordillei 

 from California eastward to the Wahsatch range, are all filled to a 

 varying depth with subaerial Quaternary accumulations. ... In each 

 one of these [Great Basin] depre rable covering of 



angular and sub-rounded Quaternary gravel, always of an evidently 

 loea! character, directly to be traced to the Banking mountain rant 

 lt> coarseness varies from large bowlders, weighiug many tons, to Hue 

 gravel, sands, and clay. Except where it lias been rearranged in the 

 now extinct Quaternary lakes, it is altogether an unstratified deposit, 

 brought down by the rush of floods from t lie flanks and caiions of the 

 mountains " (40th Par. Surv., i. 460). The sands and clays that are 

 gradually washed far forward from the piedmont tans of coarse gra\ 

 and conglomerates can hardly have been in mind when describing these 

 subaerial deposits as " altogether unstratified." I cannot find that any 



is-reference was made from the account of these heavy unstrati 

 conglomerates to the description of the almost structureless conglom- 

 erates, between ."..nun and 4,000 feet thick, already quoted from the 

 description of the Vermillion creek lake deposits (880). It is possible 

 that a re-examination of certain •■ lacustrine " conglomerates in the Rocky 

 mountain Tertiaries might lead to their explanation as arid Bubaerial 

 -its. 

 The central deposits of arid interior basins may be as fine as the 

 marginal d( are coarse. The playas or mud plains of Nevada, as 



described by Russell, and the plains around Lob Nor in the central 



