GEOLOGICAL IMPORTANCE OF SEDIMENTATION 537 



surface. But such deposits must be very shallow since as soon as 

 insolation and deflation have lowered the surrounding tracts these 

 in turn become playa basins and the waste of the former one suffers 

 removal. 



Thus ancient playa deposits may be of importance in certain 

 intermontane desert basins of the Tertiary or earlier periods, now 

 suffering stream dissection and exposure, but those of topographi- 

 cally old deserts can be of no importance except possibly as the 

 occasional surface veneer of an ancient continent, such as that of 

 the pre-Cambrian, when it passes unconformably beneath the deposits 

 formed by a marine transgression. In such an event, however, 

 they would doubtless be partly destroyed through marine planation. 



MUD-CRACKS MARGINAL TO INTERIOR LAKES 



Description. — There are many regions' of the world showing all 

 transitions between true salt lakes and fresh-water lakes with peren- 

 nial outflow. 



Where evaporation does not quite balance the inflow of water 

 there may be an occasional discharge, either at the end of the rainy 

 season or only during a series of rainy years. Such lakes without 

 being salt show fluctuating shores, usually very flat, since the water 

 does not stand sufficiently long at one level for the characteristic 

 beach slopes to form. Wide expanses, therefore, may become 

 sun- cracked and thoroughly hardened before the next rise of the 

 lake waters occurs and deposits over them another layer of clay. 



Prominent examples of this class of lakes are Titicaca in South 

 America, 80 'miles long by 40 broad, and Lakes Tanganyika and 

 Tchad in Africa. Lake Sistan in Persia with a breadth of 60 miles 

 and a length of 100 has been known to go completely dry, while 

 in occasional years during times of heavy flood it sends a stream 

 of water down the Shila. 



Nature of the geological record. — In some respects the structures 

 recorded in the formation of the successive laminae will be similar 

 to playas, but differ in that the exposed area is a transitional belt 

 between a relatively permanent water body and a permanent land 

 surface with its wind and stream-borne detritus. It is more subject 

 to wave-action, building occasional beaches; it may not be salt 



