350 STUDIES FOR STUDENTS 



developed as a land surface, and presumably by river-building rather 

 than by river erosion. In confirmation of a former lower level of the 

 Caspian Sea, Walther quotes the record of a boring on the shore of 

 the Caspian southeast of Krasnovodsk, in which dune sands were 

 found to a depth of 35 meters;' and more recently Davis has noted 

 the evidence of a low-water epoch between the Tertiary and Quater- 

 nary periods of Caspian expansion, as indicated by the stream-eroded 

 character of the present shores at Baku, which must have endured 

 for a much longer measure of time than that of the Quaternary high- 

 water stage and the present mid- water stage taken together.^ Davis 

 does not correlate the low-water stage with the development of the 

 present submerged shelf, but unless crustal warping has materially 

 disturbed the relations, it would seem a not unnatural correlation. 



Not only are interior seas thus liable to long periods of expan- 

 sion and diminution, resulting in an alternation of fluvial and lacus- 

 trine deposits, but cycles of shorter period, though enduring for 

 centuries, may be superimposed. Thus Huntington has recently 

 presented the evidence for a working hypothesis that the Lake of 

 Sistan has passed through at least ten fluvial epochs during the 

 Quarternary era ;^ the fluvial epochs being marked by widespreading 

 floods over temporary playas, leaving pink beds of clays and very fine 

 silts, often passing into layers of fine brown sand. The lake stages 

 are marked, on the contrary, by white or, more exactly, greenish clays, 

 and form but a small portion of the thickness in the exposed sections. 

 In this alternation of fluvial and lacustrine deposits the former com- 

 prise the bulk of the mechanical sediments, the latter the bulk of the 

 fossils. 



By seasonal changes in the water-level of interior seas there is an 

 especial liability to the development of a broad intermediate zone, 

 corresponding to that of the littoral bordering the open sea, and 

 characterized by variegated shales showing mud-cracks, rain-prints, 

 and ripple-marks. Thus in ancient interior basin deposits there may 

 be shore relations indicated by the nature of the strata which might 



1 "Das Oxusproblem in historischer und geologischer Beleuchtung," Petrograph- 

 ische MiUeilungen, Vol. XLIV (1898), p. 211. 



2 Explorations in Turkestan, Carnegie Institution (1905), p. 25. 



3 The Basin of Eastern Persia and Sistan, Carnegie Institution (1905), Plate X> 

 and p. 291. 



