528 STUDIES FOR STUDENTS 



on the northern flank of Mount Ellsworth.' The formation of 

 these mud-cracks was followed by a complete change of sedimenta- 

 tion at this point into the homogeneous sandstones of the Vermihon 

 cliff group, so that it is quite certain here that the mud- cracks were 

 not formed in the brief interval between two similar tidal invasions. 

 So little is known, however, of the relations between the governing 

 conditions and the characteristics of the mud-cracks that, in the 

 absence of more data, this detailed subject cannot be profitably 

 discussed, and the attention will therefore be turned to the various 

 conditions under which they originate and the associated chemical, 

 textural, and structural features which accompany them in each case. 

 Such conditions are observed to obtain first, over playas and 

 temporary lakes of arid regions ; second, upon the margins of interior 

 lakes; since the latter are peculiarly liable to seasonal fluctuation 

 in level ; third, over many river plains as a result of the periodical 

 floods in places where the surfaces are not covered with an arboreal 

 vegetation; fourth, over the higher portions of the littoral zone,, 

 where mud-flats or tidal marshes are exposed to the air sufficiently 

 long for the mud-cracks to originate. The littoral in the previous 

 article has been limited to the level which is flooded on the average 

 twice per month by the tides of the new and full moon, since above 

 this limit the tidal flooding is in a manner accidental and occasional 

 and only occurs during abnormally high tides or storms. Mud- 

 cracks will therefore be formed also over an adjacent portion of the 

 continental zone due to unusual elevations of the level of the sea. 



CONDITIONS FOR TEMPORARY PRESERVAL OF MUD-CRACKS 

 Before taking up the detailed discussion of the conditions of 

 origin, relation to other features, and final preserval of mud-cracks, 

 it is desirable to state some conclusions which the waiter has reached 

 from observation and experiment upon the conditions necessary for 

 the temporary preserval of the cracks until the stratum shall become 

 buried and form no longer the surface layer. 



Experiments were conducted first upon interstratified light brown 

 silty clay and dark brown clay of Champlain age; the former 

 smooth to the fingers, but giving a fine grit to the teeth; the latter 



^Geology of the Henry Mountains (1877), p. 9. 



