GEOLOGICAL IMPORTANCE OF SEDIMENTATION 545 



but little change of level. Even at the points of extreme change a 

 lowering of eight feet below the normal level lays bare but a narrow 

 margin, insignificant in comparison with the total area of this rela- 

 tively shallow lake. In addition it is observed that such extreme 

 conditions are never of long continuance. Therefore, until instances 

 are cited to the contrary, it must be considered that in all bodies of 

 open water the normal wave action maintains such a depth that 

 off-shore gales cannot lay bare any broad tracts of bottom. Partly 

 land-locked lagoons may in such cases run dry, but such can only 

 form a broader fringe within the actual limits of the land. The 

 border flood zone of tideless seas is therefore not so much due to 

 off-shore winds as to those which blow on-shore. Such may occa- 

 sionally flood wide belts of lowland, as is seen to take place around 

 the shores of the Gulf of Mexico. By such means in tideless seas 

 mud-cracks may originate above the normal level of the water and 

 therefore upon the land surface, but not to any appreciable extent 

 below the line of mean water. 



In the case of the Mississippi the possibilities for the formation 

 of mud-cracks are doubtless somewhat increased by the presence 

 of the mud lumps described by Hilgard, convex or low conical ele- 

 vations, sometimes 100 feet or more in diameter, showing their tops 

 at the surface. These occur in the shallow waters within one to 

 three miles of the main channel at the mouth of the Mississippi 

 River. They originate in upheavals of the soft but tough bottom. 

 Once formed they discharge mud from the top, the successive layers 

 being but a fraction of an inch thick.' These appear to be excep- 

 tional phenomena, however, and could hardly be appealed to to 

 account for the structure of extensively mud-cracked formations. 



Returning to the consideration of seas with notable tidal rangp« 

 it is doubtful if under any climatic conditions mud-cracks could be 

 made upon surfaces left bare by the tides for less than thirty-siy 

 hours; but as offshore winds may succeed for a couple of days' or 

 more in preventing flooding above the line of neap-flood tide, that 

 may be taken as the limit below which mud-cracks cannot form. 

 Taking the relative heights of the neap and spring tides above the 

 mid-tide line as 4 to 7, this gives 21.5 per cent, or approximately 



I J. D. Dana, Manual of Geology (1895), p. 197. 



