380 E. HUNTINGTON GLACIAL PERIOD IN NON-GLACIATED REGIONS 



them in the United States, Persia, Eussian Turkestan, and Chinese 

 Turkestan, and they are so described in many other places. In typical 

 cases layers of fine sand alternate with those of silt or clay; most of the 

 layers die out if followed far ; many can be seen at a glance to be lenticu- 

 lar; lenses of relatively coarse material often interrupt finer beds, and 

 everywhere, there is lack of imiformity in minor details, although the 

 formations as a whole may appear massive and homogeneous. All these 

 features, as Davis has shown in his papers on the Tertiary deposits of 

 the western United States, indicate that the strata were deposited 

 under very changeable conditions highly inconsistent with a marine or 

 lacustrine origin, but eminently consistent with a subaerial origin. Vary- 

 ing deposits of this sort may perhaps be laid down in the littoral zone of 

 estuaries ; but such a supposition is out of the question in the cases under 

 consideration, because of the vast extent of the red deposits and because 

 they are not found to merge into marine deposits. The red strata of 

 central Asia appear to extend over an area 1,000 miles long from east to 

 west. 



Other evidences of subaerial origin are found in common features such 

 as mud-cracks, ripple-marks, rain-drop prints, and the tracks of terres- 

 trial animals. These indicate that the regions of deposition were flooded 

 at intervals, but were exposed to the air a large part of the time. Bar- 

 rell* has shown conclusively that only a small fraction of such markings 

 can have been formed upon tidal flats ; the great majority must have been 

 formed in deltas, playas, and flood-plains, chiefly in arid regions. 



2. FEATURES INDICATING ORIGIN UNDER ARID CONDITIONS 



Having concluded from the features mentioned above that many red 

 strata are subaerial in origin, we must inquire into the evidence as to the 

 kind of climate which prevailed at the time of their formation. It is well 

 known that in the world today suliaerial deposition is in progress chiefly 

 in arid regions, and therefore, on general principles, subaerial strata of 

 former times, whether red or of some other color, are likely to have been 

 formed in arid regions. The red beds, however, contain direct evidences 

 of aridity. One of the commonest and most universally recognized is 

 intercalated beds of gypsum, indicating shallow, saline lakes. Another 

 is eolian cross-bedding of a type which can not, apparently, be produced 

 by water. Its chief characteristics are the comparative uniformity in size 

 of the sand-grains in which it is found, the frequent tangency of the 

 individual beds to the floor on which they lie, and the large scale of the 



* Joseph Barrel! : Relative geological importance of continental, littoral, and marine 

 sedimentation. Journal of Geology, vol. xiv, 1906. 



