EROSION IN ARIZONA BOLSON REGION 159 
due to the drying of a mud sheet show extreme development here. 
Wind, sand, and salt often comprise the filling of the mud cracks. 
Mud crack tubes. At the margin of the Salton Sea the drying-out 
of irregular mudcracks develops a surface crust of salt, arching over 
the crack. When well developed these become perfect tubes, and 
simulate the salt crusts deposited around small roots, or even fossil 
tubes of worm burrows. Shallow channels caused by wind erosion 
were excellently developed on the Salton beaches, and might find 
preservation in fossil playa deposits. Where the playa develops 
alternate extended and lacustrine conditions, restricted with well- 
developed beaches, dune formations, and blown-sand deposits, with 
plunging and truncated structure, invade and cover the playa muds. 
Special development of dune and standing-water action was noted in 
the area of the Crescentic Dunes, Carrizo Sands, Salton Basin, which 
explains some irregular structures showing fragments and lenses of 
wind sand in shale, noted especially in the Painted Canyon series, 
Mecca, and deserves, perhaps, at least a passing notice. Between the 
individual dunes, the ground is strewn with rock fragments, composed 
largely of pieces of strongly cemented dune sand, and protruding above 
the surface is a most irregular wind-sand formation, also cemented. 
Its irregular truncations and bevelments showed plainly that it was 
built up of truncated bottom layers of dunes that had passed over, 
leaving behind their bottom layers, captured by the salt seepage 
water, and later cemented by limonite, the whole formation showing 
the extreme conditions under which it was formed.' 
Animal remains in playas.—Judging from the large number of 
bogged cattle that perish in the Arizona “cienegas’’ (fresh-water 
mud flats) during periods of drought, the same ought to be expected 
in older playas. Salt water would not attract the animals. The 
wind-rows of decaying fish that occur after flood intervals, in the 
Laguna Maquata, described and explained in the following quotation, 
are suggestive in connection with the development of petroleum, in 
strata deposited in arid regions suffering from periodic invasion by a 
strong river or the sea. 
This low ridge cf dead fish was seen to extend for about 15 miles and may 
have been double that length. A similar observation was made by Orcutt in 
t Tolman, Joc. cit. 
