EFFECT OF DESICCATIOK 



911 



and the sediments cut by them, when formed on the seashore within the 

 tide limits, are a product of marine conditions and belong logically to the 

 phenomena of marine sedi- 

 mentation. Under favorable 

 conditions they may differ 

 from those formed in fresh- 

 water mud as definitely as do 

 the fresh-water and salt-water 

 shells. Experiments in desic- 

 cation carried out by the 

 writer have shown that the 

 polygons formed in fresh- 

 water mud have a marked 

 tendency to warp up at the 

 margins (figure 4). In saline 

 mud the pol5^gons remain flat 

 or warp downward at the 

 edges. The latter tendency 

 has been noted only in very 

 saline muds. Figures 5 and 6 

 show the marked contrast in 

 the behavior of desiccated salt and fresh-water mud. 



In the case of fossil mud- 

 cracks the geologist can make 

 definite deductions regarding 

 the salinity of the original 

 mud only where there has 

 been distinct upwarping or 

 downwarping of the polygons. 

 Where the surface is flat, as is 

 usually the case, lack of warp- 

 ing might as likely be due to 

 the tenacity of the mud over- 



FiGURE 5. — Mud-crack showing Polygons slightly 



downwarped at the Margins 

 A characteristic of mud-craclv in very saline mud 



the 



upwarpmg ten- 



dency characteristic of fresh- 

 water mud-crack polygons as 

 to the normal tendency of the 

 salinity of the sea-water to 



Figure &.— Fresh-water Mud-crack with Polygons produce flat polygons. Where 

 Showing an extreme degree of upwarp the polygons show a definite 



saucer - like upv^arp at the 

 margins, however, the inference that they were formed from fresh- water 

 mud and represented contijiental sedimentation would be inevitable. I 



