GEOLOGICAL IMPORTANCE OF SEDIMENTATION 547 



As a final exception may be noted the effect of ttie previously 

 mentioned monsoon winds, as seen on the Runn of Cutch, southeast 

 of the delta of the Indus. In this case winds blowing steadily on-shore 

 for months at a time raise the sea-level sufficiently to flood with sea- 

 water large tracts of marshy country which during another portion 

 of the year become an arid desert. Such conditions are, however, 

 very exceptional and probably are most likely to occur broadly 

 where rivers have previously built up alluvial plains, so that this 

 seasonal extension of the littoral zone may only take place in con- 

 nection with the continental deposits of rivers. 



As the season of off-shore monsoon winds, during which the mud- 

 cracks form, should be normally a season of aridity, it is likely that 

 saline deposits from evaporated sea-water should frequently be 

 associated with the mud-cracks. This is notably the case in the Runn 

 of Cutch. The two features are also associated in the saline beds 

 of New York. If the climate on the contrary is a pluvial one, the 

 rain which would wash off the residual sea-water would also prevent 

 the formation of mud-cracks from the sea- water as a cause. 



To sum up: it is seen that mud-cracks are confined to an upper 

 fraction of the httoral zone, and where occasionally formed beyond 

 it by inundations of the sea only attain a broad development at the 

 present time in arid regions where continental river deposits have 

 "been previously built. In this case the mud-cracked strata should 

 be at least frequently saliferous. 



Nature of the geological record. — The nature of the geological 

 record and the features which distinguish mud-cracks of the littoral 

 zone from those made under other conditions may be gathered from 

 the preceding discussion of the conditions of present occurrence. 

 ■ The zone itself marks the transition between a subaerial and a sub- 

 aqueous surface, in the case of deltas each nearly horizontal but at 

 different levels. When the delta deposits of these three regions 

 are seen in cross-section, the littoral will be a transition belt between 

 continental and marine deposits. As the seashore during the accu- 

 mulation of the strata was ordinarily a shifting line, as seen in cross- 

 section, it will pass nearly horizontally between the two. If the 

 subsiding land was receiving no river deposits, the lower surface will 

 be an erosive surface represented by an unconformity. If the land 



