GEOLOGICAL IMPORTANCE OF SEDIMENTATION 531 



even where the crack fails to reopen, by the interruption of the faint 

 lines of stratification at the margins and a slightly marked weakness 

 along this line. When this mud was beaten up with the water, 

 dried quickly over gas and then recovered with water, the air rapidly 

 escaping from the many minute bubbles produced an audible sim- 

 mering, and the mud, orio;inallv so retentive of its form, soon fell 

 into a mush. It was also observed that the dry natural clay was 

 subject to a slight exfoliation upon rapid wetting. These facts 

 point out the importance of close texture, obtained by slow sub- 

 sidence and long standing under water before drying. This requires 

 the moisture to be transmitted slowly, by capillary action, and allows 

 the mass to expand as a whole. 



The conclusions from these observations and experiments will 

 doubtless be somewhat modified by more extensive observations in 

 suitable regions, but may be preliminarily stated as follows: 



A mud-cracked loam or silty clay, even when the sand particles 

 are imperceptible to the fingers, is an unfavorable material for the 

 preservation of its detailed surface structures, except possibly when 

 remaining moist, so that but little swelling and exfoliation take place. 

 Upon being wet by rain, the rapid swelling and disintegration of the 

 surface stratum would turn the surface of such a deposit into a 

 creamy mud, which, if remaining in situ, might preserve upon 

 redrying blurred impressions of the previous cracks and other larger 

 surface features, but which would be peculiarly liable to be swept 

 away and intermingled with the detritus of the following flood 

 similar to the one which left the material. Even if the flood sweeps 

 down from the mountains upon previously unwet desert plains, the 

 few minutes of wetting necessary would suffice to destroy the detail 

 of surface features before a sufficient new layer of sediment could 

 be laid down. This would seem to explain the mud-lakes into which 

 many playas are transformed during the rainy season. Upon such a 

 formation becoming lithified, the record of mud-cracks might be 

 greatly masked, if not entirely obliterated. The development of 

 joint planes, and even a faint cleavage, would add to the difficulty of 

 detection. Where the finer details of the original surface are pre- 

 served, however, or the sharp surfaces of stratification between 

 unlike laminae, it would seem impossible that the mud-cracks could 



