DAVIS. — ROCKY MOUNTAIN TERTIARIES. 369 



basin of Asia, as described by Svea Hedin, are composed of exces- 

 sively fine materials, yet they are rather of fluviatile than of lacustrine 

 origin in the ordinary sense of these words. If a shallow lake occurs 

 in the lowest part of interior basins of this character, it occupies but a 

 small part of the entire depression ; it is variable in position, shifting to 

 a new site as it is driven about by growing deltas; it is still more vari- 

 able in volume, changing with the weather, the season, the century, and 

 the climatic cycle. Only in epochs of moist climate does a playa lake 

 reach dimensions comparable to those attributed to the Tertiary lakes 

 of the Rocky mountain region ; yet in dry as well as moist epochs, 

 fine sediments gather in the basin, aggrading its floor. The manner in 

 which the fine mud of playas is distributed when the very shallow water 

 is agitated by wind is said to prevent the production of well defined strati- 

 fication. Ancient playa deposits would therefore be prevailingly of a 

 massive structure, instead of being finely laminated like typical lake 

  beds ; and they would be associated with wedges of coarser deposits that 

 were washed forward on the basin floor by intermittent streams. 



15. kalian Deposits. — Wind-borne dust and showers of volcanic ashes 

 are both of importance as possible contributors to subaerial deposits, 

 particularly in regions where the streams are aggrading the surface, 

 ana where running water is incompetent to remove the aerial sedi- 

 ments. The recoCTuitioa of wind-borne dust is still a matter of uncer- 

 tainty, or at least a subject of disagreement. Volcanic materials are 

 ill recent years generally detected by the aid of the microscope. 

 Matthew has recently called attention to the importance of wind de- 

 l)Osits in an article entitled, " Is the "White River Tertiary an JEolian 

 Formation?" (Amer. Nat. xxxiii, 1899, 403-408). This author begins 

 by saying that the Loup fork beds have been " shown to be largely a 

 flood-plain deposit " (403), and closes with a statement that the Rocky 

 mountain Eocene is " probably a mixture of lake and fluviatile sediment 

 — what proportion of each would not be easy to determine" (408). He 

 objects to the origin of the White river clays in a lake because of the 

 size of the lake required, and because of the absence of an eastern 

 barrier and of shore lines ; furthermore, he states that the clays are 

 not well stratified ; they contain land mammals in abundance, but they 

 preserve no plants, no fish, and no aquatic reptiles or invertebrates. 

 While the included sandstones are thought to be river-laid, the clavs 

 are said to be better accounted for by aeolian action, such as is now 

 going on in the production of loess on the open grassy surface of the 

 subarid Plains. This article is of especial interest, not only from its 

 VOL. XXXV. — 24 



