FACTORS AFFECTING DEVELOPMENT OF MUD-CRACKS 139 



partially dried mud (see Fig. 4). The cracks developed with the 

 retreat of this zone away from the area which first dried. In a 

 small patch of this first dried section no mud-cracks formed. 

 This experiment shows that approximately parallel mud-cracks 

 may be developed by differential desiccation, and affords a clue 

 to the cause of certain kinds of joints which appear to be definable 

 as parallel mud-cracks of considerable vertical extent. 



Fig. 4. — Mud-cracks cutting the mud into ribbon-like strips. \ natural size 



Experiment 4.— A 2-quart mixture of blue clay and water was 

 divided into two equal parts. A tablespoonful of salt was added 

 to one of these, and the other was left fresh. The two mixtures were 

 placed in shallow pans 9^ inches in diameter and put in the sun for 

 evaporation and desiccation. Complete drying or desiccation of 

 the salt-water pan was finished on the eighth day after starting this 

 experiment. The first noted difference between the two pans was 

 the earlier drying out of the saline mixture. All the liquid water 

 had left the salt-water mixture at least a day before the fresh-water 

 mud had ceased to be a semi-liquid mass. The desiccation was 

 finished in a temperature of 1 io°. 



