PALUDAL AND LACUSTRINE CLASTICS 605 



will in time be covered by the shifting sands of the desert and a 

 relief mold will be produced by the covering sands. Such a relief 

 impression will be more readily preserved than the original im- 

 pression, which may be destroyed by the softening of the clay 

 surface of the playa. A single extensive inundation by heavy rains 

 of a desert surface may permit a wide horizontal migration across 

 this surface of animals which never before and never after entered 

 this region. Thus their tracks may be widely preserved in a single 

 horizon in a desert formation, like those of Cheirotherium in the 

 Upper Buntsandstein (\\'alther-57 :5'(S'), even though the animal 

 lived during a much longer time period. Repeated floodings. an- 

 nually or at intervals of many years, will permit the formation of 

 successive track-bearing layers, by animals living on the border of 

 the desert. This mode of preservation certainly accords best with 

 the characters of the tracks found in such formations as the Newark 

 sandstone of the eastern United States, whereas the frequent as- 

 sumption that the successive track-bearing beds were made between 

 tides and buried by the sediment brought in by the returning tide 

 does not allow for the obliterating efifect of the tide, an argument 

 equally applicable to stream-laid deposits upon the fresh tracks. 

 (\^oigt-54:7(5(5.) 



The conditions favoring the preservation of footprints in desert 

 regions militate against the preservation of the animals themselves. 

 For, unless the body is buried at once, it is sure to fall a prey to 

 the desert carnivora, while sun and wind will complete the destruc- 

 tion of what remains. This explains the scarcity of remains of the 

 animals in the strata which contain their footprints. 



Other Structural Characters. Ripple marks and rill marks, 

 though usually regarded as typical only of marine formations, are 

 equally, if not more, characteristic of the non-marine deposits. 

 Their discussion is. however, deferred until hydroclastic sediments 

 have been more fully discussed. 



NORMAL PALUDAL AND LACUSTRINE CLASTIC 



DEPOSITS. 



Clastic deposits in swamps (paludal elastics) are of relatively 

 little importance except when associated with vegetal deposits. 

 These latter are by far the most important, the elastics being subor- 

 dinate and confined to the sediments carried in by wet weather 

 streams and rains, or the dust settling out of the air. 



Deposits in ponds and lakes, on the other hand, i. e., in water 



