366 E. HUNTINGTON — GLAClAL PERIOD IN NON-GLAdtATED REGIONS 



aerially by occasional widely spreading floods, or in temporary playas, 

 during periods of extreme aridity. 



3. White or light-colored lacustrine clays. — The wMte or, more ex- 

 actly, the greenish clays, are very different from the red layers with 

 which they are interbedded. On the upper and lower edges they are 

 mixed with fine sand or are more or less banded. Occasionally a purple 

 layer occurs or a band of yellow clay and fine sand in which are fossil 

 leaves and reeds. The main mass of each stratum, however, consists of 

 solid, unbroken layers of pure clay, uniform in texture and color and 

 showing none of the slight variations characteristic of the pink beds. 

 The color indicates that the materials were brought quickly from their 

 place of origin among the mountains and have never been long exposed 

 to erosion, or else that whatever oxidization has taken place occurred in 

 the presence of plants capable of removing the oxidized iron. Which- 

 ever may be the case, the green strata as a whole show no sign of sub- 

 'aerial origin and appear to be typically lacustrine. 



CLIMATIC SICNIFJCANCE OF THE ALTERNATING RED AND WHITE STRATA 



OF 8EYI8TAN 



The red or pink and the white or green beds differ from one another 

 chiefly in the conditions of deposition. The material of the clayey portions 

 of the pink appears to be identical with that of the green, except that it 

 contains a large amoimt of oxidized iron. It is reasonably certain that 

 both pink and green were derived from the same source. The alternation 

 of such subaerial and subaqueous beds indicates that during the most 

 recent geological times the lake of Seyistan has alternately retreated 

 from and encroached upon large areas of its quondam bed. The dura- 

 tion of each epoch of retreat and encroachment must have been consider- 

 able, for the accumulation of from 5 to 20 feet of the finest clay must 

 have taken some thousands of years, if deposition was formerly as slow 

 as it now appears to be in the Hamun-i-Seyistan at a distance from the 

 river mouths. The change from subaerial to lacustrine conditions and 

 the reverse must have been gradual, for the pink and green deposits often 

 shade into one another. The sandy layers mixed with the upper and 

 lower portions of the lacustrine clay seem to indicate shore conditions, 

 and the layers of yelloAv or purple clay with the inclosed fossil plants 

 point to the existence of marshes, like the swamps of today, on the shores 

 of the retreating or advancing lake. 



When we attempt to explain the variations in the lake, four hypotheses 

 present themselves : First, the lake may have had an outlet which was 

 repeatedly dammed by volcanic eruptions or otherwise, and as frequently 



