368 PROCEEDINGS OF THE AMERICAN ACADEMY. 



dissolved from the surface, transported in solution by the torrential 

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

 great incrustations around the margins of the bolson plains of northern 

 Mexico " (18th Ann. Rep., U. S. G. S., 256). 



Many other references might be made to the importance of the sub- 

 aerial deposits or " wash " at the base of sub-arid mountains, for such 

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

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

 Southern California, Bull. Geol. Soc. Amer., iii, 127 ; Manual Geol. 

 of India, 2nd edition, 417, 418 ; Blanford, Superficial Deposits in the 

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

 1873, 493), 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 ranges, 

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

 posits that are formed by storm floods near the mouths of mountain 

 valleys. For example: — "The interior valleys of the Cordilleras, 

 from California eastward to the "VVahsatch range, are all filled to a 

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

 one of these [Great Basin] depressions is a considerable covering of 

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

 local character, directly to be traced to the flanking mountain ranges. 

 Its coarseness varies from large bowlders, weighing many tons, to fine 

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

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

 brought down by the rush of floods from the flanks and canons of the 

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

 gradually washed far forward from the piedmont fans of coarse gravels 

 and conglomerates can hardly have been in mind when describing these 

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

 cross-reference was made from the account of these heavy unstratified 

 conglomerates to the description of the almost structureless conglom- 

 erates, between 3,000 and 4,000 feet thick, already quoted from the 

 description of the Vermillion creek lake deposits (380). It is possible 

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

 mountain Tertiaries might lead to their explanation as arid subaerial 

 deposits. 



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

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

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



