552 STUDIES FOR STUDENTS 



of mud-cracks is due to an inheritance of expression from tlie past 

 or if undue importance has been given in the present paper to mud- 

 cracks of continental, and especially of flood-plain, origin in arid 

 and subarid climates. 



In both LeConte's and Scott's textbooks no mention is made 

 of mud-cracks originating by any other means save by the laying- 

 bare of tidal flats, and in both it is used as an argument proving the 

 estuarine nature of the Newark basins; and Scott discusses the 

 question whether the basins were parts of one or two "continuous 

 bodies of water," p. 445. 



Recently Huntingdon' has spoken of sun-cracks and ripple- 

 marks taken in connection with other features as initiating conti- 

 nental sedimentation of the Tertiary in Central Turkestan. 



It appears as though, after the retirement of the sea, the land was covered 

 with great playas, on which water first stood in thin sheets, forming ripple-marks 

 in the mud and then retired or was evaporated, allowing the surface to become 

 sun-cracked. As time went on streams began to flow across the playas, at first 

 slow and broad and able to cut only shallow channels which were afterward 

 filled and covered, assuming the form of very thin lenses of a material slightly 

 different from that of the surrounding playa strata. Then, as the strength of 

 the streams increased, sand was deposited over the whole area, and the channels, 

 now deep and distinct, were filled with gravel. Lastly gravel was deposited 

 almost everywhere. 



So far as the writer is aware the only attempt at a discussion of 

 the several methods of origin of mud-cracks and the relative chances 

 of their preserval is by Penck,^ who points out that they, as well 

 as foot-prints and rain-prints, occur on sea-beaches, over the flood- 

 plains of rivers and the shores of interior seas, but that the surface 

 bearing the markings must to a certain degree have hardened and 

 consequently have remained as a land surface for a certain length of 

 time in order that the impressions should not be washed out by the 

 next invasion of waters. For that to be accomplished he states that 

 the sea-coast is less favorable than the flood-plains of rivers and 

 the margins of lakes. In the latter cases the exposed floor dries 

 for weeks or months and attains a considerable hardness before 

 being again overflowed. 



I "Explorations in Turkestan," Carnegie institution of Washington (1905), 

 pp. 164, 165. 



- Morphologie der Erdoberfldche (1894), Vol. II, pp. 25, 26. 



