144 E - M. KINDLE 



the tenacity of the mud, which on a mud flat may prevent the top- 

 most cracked layers from partially splitting away from the sub- 

 jacent layers, as they must do if this warping occurs. Clearly the 

 cohesion between the layers of mud is greater than that between 

 the smooth bottom of the pan and the mud in it. Observation of 

 sun-cracked fresh-water mud on the bottom of evanescent ponds 

 will show that the polygons warp upward or remain flat, according 

 to the tenacity of the mud. Where the tenacity of the mud is 

 slight, the saucer-shaped polygons are dominant. 



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 warping is 

 as likely to be due to the tenacity of the mud overcoming the warp- 

 ing influence of fresh water as to the normal influence of the salinity 

 of sea water. Where the polygons show a definite saucer-like 

 upwarp at the margins, however, the inference that they were 

 formed from fresh-water mud would be inevitable. I have de- 

 scribed 1 from bed A of the Mount Wissick section in New Bruns- 

 wick an example of this kind which in the light of these experiments 

 must be referred to continental or fresh-water conditions, although 

 I originally supposed it to have been formed on a tidal flat. 



Fossil examples of the inverted-saucer type of polygon due to 

 the drying of very saline muds are apparently not very common. 

 Some peculiar structures in Silurian dolomite described by Gilbert 2 

 and illustrated 3 by Kindle probably represent a phase of this 

 phenomenon. These curved plates in the Lockport dolomite near 

 Niagara Falls, which are probably the result of the desiccation 

 of highly saline sediments, were supposed by Hall to be of concre- 

 tionary origin. They immediately precede in the section a rock 

 series in which beds of gypsum and rock salt afford conclusive 

 evidence of the highly saline character of the sediments deposited a 

 little later. 



1 Geol. Surv., Can. Mns. Bull. 2, 1914, P- 37- 



2 " Undulations of Certain Layers of the Lockport Limestone" (Abstract), 

 Science, N.S., XXI (1905), 224. 



3 U.S. Geol. Surv. Folio No. igo, 1914, p. 59, PI. 24. 



