GEOLOGICAL IMPORTANCE OF SEDIMENTATION 527 



or limy deposits which upon drying shrink notably in volume. De- 

 posits whose grains are predominantly of sand size cannot give rise 

 to mud-cracks, since when wet the water stands in the pores, and 

 the deposit does not lose markedly in volume upon drying, but only 

 in weight. This limitation in regard to the necessary size of par- 

 ticles requires that the deposit should originate under very quiet 

 waters, which are either removed by evaporation or slowly drained 

 away with bottom velocities of less than a third of a mile per hour, 

 since such a current will lift fine sand. 



Mud-cracked surfaces are observed to vary much among them- 

 selves. Sometimes they inclose polygons a few inches across, some- 

 times a foot or more in diameter. In depth they may terminate 

 within a few inches or they may pass downward as many feet. As 

 factors governing the nature of the mud cracks may be mentioned 

 the shrinkage ratio of the deposit; the porosity by means of which 

 water may be conveyed upward by capillarity, tending to prevent 

 shrinkage; the varying nature of the stratified deposit, the cracks 

 not being able to pass through thick strata of sand ; the thoroughness 

 of saturation; the length of the period of desiccation; the tempera- 

 ture and dryness of the air. It seems that a thorough observational 

 and experimental study should throw important light upon some 

 of these relations by means of which certain of the conditions attend- 

 ing the formation of mud-cracks in ancient strata could be recognized. 



It is possible that a definitive solution is not usually to be obtained 

 on account of the number of the governing factors. But the solution 

 should be narrowed to one of two or three alternatives. For instance, 

 providing that an' argillaceous formation is homogeneous through- 

 out, the depth of the crack will probably vary roughly as the square 

 of the time of desiccation. It will also vary with some power of 

 the temperature measured on a centigrade scale, and with some 

 power of the degree of dryness of the air. A knowledge of these 

 values would enable one to say if certain mud-cracks could have 

 been formed in the fortnightly interval between spring tides, or if 

 a season or more of desert heat and dryness were necessary. 



As a striking example may be cited the mud-cracks described 

 by Gilbert which penetrate ten feet downward into the variegated 

 shales of the Upper Shinarump of the Jura Trias whel-e exposed 



