16 SHRINKAGE FISSURES RECENT DEPOSITS. 



along joints or breaks in the drift, and thus form 

 hollows with perpendicular sides; but in such cases 

 the shape of the sides is generally soon modified by 

 meteoric abrasion. 



Something may also be learned from railway and 

 other embankments, as they often subside on account 

 of the ground under them being weak. If the 

 foundation is bog, the peat will bend down at first 

 towards the embankment, but afterwards it will 

 crack in lines rudely parallel, and eventually form 

 fissures more or less wide, according to the depth the 

 embankment sinks. Gravel, in general, when it 

 gives way, acts very similarly to bog, bending down 

 to the embankment, and eventually cracking and 

 forming fissures. So also do some clays, but others 

 act like mud, which squeezes out from under the 

 embankment, and rises in curves. Clay will stretch 

 to a considerable extent, and may remain unbroken ; 

 but mud seems to crack immediately at first, per- 

 haps, often irregularly, but subsequently, when it has 

 lost most of its moisture, regular fissures may be 

 formed in it. Subsidences may bend the different 

 strata, but in many cases they occur along a joint, 

 and thereby break the continuity of the beds or 

 strata, thus changing a joint into a dislocation or 

 fault. 



