SHRINKAGE FISSURES RECENT DEPOSITS. 13 



shed from the effects of heat and wind or contact with 

 water ; yet in some cases what was originally a crack 

 may eventually become a watercourse. 



When a section is laid open in a bank by a river, 

 or lake, it is usually found to be stratified, according 

 as materials of different composition, colour, or 

 texture may have been deposited in layers or strata. 

 In such accumulations two distinct kinds of cracks 

 or joints are formed ; one set (minor joints) are local, 

 and only affect a stratum, or two or more associated 

 strata, while the other (master joints) cut through 

 all the layers. The minor joints are, in general, 

 locally systematic that is, in some layers, only, they 

 will all be regular, and in others all irregular. The 

 master joints on the surface are usually more or less 

 regular, but beneath they may be either regular (c, d, 

 fig. 3, PI. I), or irregular (a, c, f, b, fig. 3, PI. I), 

 as they take advantage of the weakest lines in each 

 successive stratum they pass through ; or even at 

 times they may for a while run along the stratifica- 

 tion, and if there be an open crack at the surface, 

 it may not be continuous downwards, the lower part 

 of the fissure being either to one side or the other of 

 the upper (fig. 3, PI. I.) 



If two master joints cross, they form a greater or 

 less vacancy at the intersection (fig. 4, PI. I.) ; but 

 if three or more cross at one point, the opening will 

 necessarily be larger, not only on account of the com- 



