FACTORS AFFECTING DEVELOPMENT OF MUD-CRACKS 143 



polygons in which the upper and lower surfaces were perfectly flat, 

 the edges showing no inclination either to warp up or down. 



SUMMARY AND DISCUSSION 



The experiments described above justify the following deduc- 

 tions: Rapid desiccation produces mud-cracks which are more 

 widely spaced than those produced by slow desiccation. In mud- 

 cracks occurring in rocks of the same or similar composition the 

 relative size of the resulting polygons would therefore serve as a 

 basis for inferring the relative temperatures under which they were 

 formed. 



The composition and the resulting tenacity of the mud very 

 materially affects the spacing of the mud-cracks. The presence 

 of marly material or the addition of sand gives polygons which are 

 much smaller than those formed in clay mud (Figs. 1-3). In the 

 case of sandy mud a sufficient excess of sand entirely prevents the 

 formation of mud-cracks. Hence a bed of sand might be exposed 

 to subaerial conditions without furnishing mud-crack evidence 

 of the fact. Temperature and tenacity of the material are two 

 primary factors in controlling the spacing of mud-cracks. 



Approximate parallelism of mud-cracks may result from zonal 

 drying of the mud. The parallelism seen in many systems of joint 

 structure may thus be duplicated under special conditions in 

 shrinkage-cracks in mud. 



A high degree of salinity delays the formation of mud-cracks 

 and results in polygons in which the margins are inclined downward 

 (Fig. 6). These are in marked contrast to the polygons formed in 

 fresh-water mud, which dish upward, saucer-like (Figs. 1, 2, 3, 

 and 5). The polygons formed in mud with the salinity of ordinary 

 sea water warp neither upward nor downward at the margins, 

 but retain a flat surface. It should be pointed out here that the 

 marked differences observed in the experiments between the be- 

 havior of fresh-water, highly saline, and moderately saline muds 

 are not ordinarily so well marked in nature as the accompanying 

 illustrations might lead the reader to expect. The strong tendency, 

 as shown by the pan experiments, of fresh-water mud to warp 

 upward and of very saline mud to warp downward at the margins 

 of the polygons on cracking is modified and often neutralized by 



