GEOLOGICAL IMPORTANCE OF SEDIMENTATION 351 



readily be mistaken for those of an epicontinental sea in tidal con- 

 nection with the ocean. As distinguishing features it may be noted 

 that in the former case the subaqueous shore deposits take place con- 

 tinuously for months. The periods of exposure likewise endure for 

 months, and the prolonged desiccation gives a maximum opportunity 

 for the formation of mud-cracks and the hardening of rain-prints or 

 foot-prints. This hardening is favorable to their preservation, as 

 Lyell has shown, since the record is not readily washed out by the 

 returning waters, which bring with them another layer of sediment^ 

 by which the record is buried and indefinitely preserved.^ 



The shores of the epicontinental sea, on the contrary, if open to 

 the tides, are subject to two daily inundations, and the chances for the 

 formation and preserval of mud-cracks is at a minimum, except at 

 the highest limits of tidal flooding. 



Variable water-level — marine deltas. — To consider the effects of 

 irregular vertical movements upon deltas facing the ocean waters, it 

 is to be noted that the change is due to movements of the land rather 

 than of the sea. Of land movements the nature of subsidences is 

 more difficult to observe than that of elevations. In the latter, suc- 

 cessive beach-lines carved along coasts, successive development of 

 partial peneplains, and the actual observations of uplifts, as those 

 noted during the nineteenth century along the coast of Chili, all give 

 testimony to the largely intermittent character of upheaval. But in 

 regard to subsidences also, observations have been made. These are 

 sometimes slow, and for a time at least equable, as that affecting the 

 present eastern coast of the United States, at other times subsiding at 

 unequal rates, as indicated by the halt during the past century of the 

 subsidence of the deltas of the Rhine and the Meuse, compared with 

 the occasionally disastrous inroads made during the previous millen- 

 nium. ^ Again at times districts suddenly subside during earthquakes. 

 Such movements must tend to occasionally flood considerable por- 

 tions of the delta surfaces of large rivers with sea water, which will 

 stand over the regions perhaps for centuries or millenniums before it 

 is again reclaimed by delta-rebuilding. 



Perhaps the most striking instance is to be found in the Runn of 



' Quarterly Journal of the Geological Society 0} London, Vol. VII (1851), p. 239. 

 ' A. de Lapparant, Traite de geologic, 4th ed., pp. 570, 572. 



