158 C. F. TOLMAN 
Stratification.—The stratification in outwash deposits is especially 
developed. The alternate floods, of varying volume, now carry 
mud and now boulders, both depositing. Also the wind action 
described sorts out a protective layer of larger pebbles. The crusts 
develop a more or less stratified form, especially where the surface 
is level. This is, however, modified by a dome structure, due to the 
drying-out of the slightly elevated portion, causing a capillary flow 
in that direction, and accentuated by a slight erosion of the hollows. 
Playa deposits—The deposits of the playas vary considerably 
especially as they are formed (1) under a water sheet of moderate 
depth; (2) in a thin sheet as an evaporating mud surface; (3) as a 
flood-sheet deposit from a strong flow from above; and (4) these 
are all modified by wind erosion during periods of aridity. The 
strata deposited under lacustrine conditions need not be considered 
here, as the details of such deposition have been under geological 
observation for many years. An excellent summary of the details 
is found in Barrell’s articles, cited above. Summarizing some of 
the points of especial import, the following are noted: (1) The 
strands and terraces of inland water sheets comprise some of the 
first of the climatic criteria discovered by geology; (2) coarse 
material is distributed in belts, parallel to the strands, and found 
only in their vicinity; (3) the deeper muds are largely wind-borne, 
and therefore of even fineness; (4) lack of subaerial markings of all 
kinds, within the body of the deposit, as well as of oxidation, are the 
chief features separating lacustrine from playa and subaerial delta 
deposits; (5) thick beds of pure sorted mica, floated out into deep 
water (observed in the Painted Canyon series, Mecca, California), 
indicate deep-water deposition under arid conditions. 
On account of the lack of dissection of the playas visited, less data 
have been gathered on this subject than on the others.’ Some of the 
characteristics have been inferred from observations on playas in the 
making. 
Chemical deposits of the playas.—The chemical deposits of the 
playas are the more soluble salts, while that of the lower portion 
of the surrounding bajada is largely carbonate of lime. 
Playa mud. Subaerial characteristics—The well-known markings 
« For characteristics discovered by Huntington in Turkestan, see op. cit., p. 286. 
