CHANGES OF CLIMATE OP SEYISTAN 367 



swept clear; second, a large tributary may liave been alternately diverted 

 to and from the lake ; third, the region of Seyistan may have been sub- 

 jected to a highly specialized type of rhythmic earth-movements whereby 

 the lake was poured from side to side or its bottom was repeatedly raised 

 and lowered; and, fourth, the Pleistocene strophic period may have con- 

 sisted of a much larger number of epochs than has been commonly sup- 

 posed from a study of glaciation. Only the fourth hypothesis appears 

 tenable, as I have shown in "Explorations in Turkestan." A succession 

 of ten arsial epochs of lake expansion alternating with ten thesial epochs 

 of lake contraction would accoimt for all the observed facts of Seyistan. 



INDICATIONS OF LATER CHANGES OF CLIMATE AT SEYISTAN 



The history of the climate of Se3'istan after the time when the clays of 

 the old lake floor were uplifted seems to be continued in three thick 

 deposits of gravel alternating with fine silt. These cap the clays in 

 many places, as appears in figure 4. The gravels and silts seem to be 

 related in the same way as the alternating clays which underlie them, 

 the gravels corresponding to the pink clays, and the silts to the green. 

 The gravel and silt are coarser and more largely subaerial than the clays, 

 apparently because the eartli-movements already mentioned uplifted this 

 northwestern part of Seyistan and steepened the grade of the streams. 



To the three arsial epochs indicated by the gravels and the ten indi- 

 cated by the clays we must add two more to account for two strands about 

 15 and 25 feet above the present average high-water mark. In order to 

 explain all the phenomena of the lake of Seyistan, we are therefore 

 obliged to postulate 15 cycles, each with its arsis and thesis. We rebel 

 at the thought of adding cycle to cycle in this wholesale fashion; yet 

 fifteen or a hundred cycles are as reasonable as one or two. Putting 

 together all the evidence of clays, gravels, and strands, it appears that in 

 eastern Persia the last part of the Tertiary era and the whole of the 

 Quaternary form a strophe characterized by an extraordinary series of 

 climatic oscillations. At first the extremes appear to have been mild or 

 brief, then more severe or longer, and now again mild. The epochs suc- 

 ceeding the maximum appear to correspond to those known elsewhere as 

 characteristic of the Glacial period. Although the correlation has not yet 

 been perfectly established, the lacustrine strands and the gravel deposits 

 of Seyistan appear to be synchronous with the series of fluvial terraces 

 found universally in Persia and Turkestan. The terraces, as has been 

 said, can be correlated with the moraines of central Asia, and there is 

 no reasonable doubt that the moraines are synchronous with those of 

 Europe. 



XXXII — Bull. Geol. Soc. Am.^ Vol. 18, 1906 



