SHRINKAGE FISSURES RECENT DEPOSITS. 15 



while subsequently cracks form in the superincum- 

 bent mass, till eventually, having lost all support, it 

 slides away seaward. On this coast such subsidences 

 are usually not of very great extent, but in a few 

 cases remarked they were considerable. 



Streams may act somewhat similarly to the sea; 

 and examples of their undermining and lowering 

 ground may be studied on the south slopes of the 

 mountain group called Slieve Arra, County Tipperary. 

 On these slopes there is a large accumulation of 

 boulder drift, which in places contains subordinate 

 beds of running sand. In the boulder drift the rain 

 and rivers have opened deep ravines, and if these 

 reach down to the sand, it runs, thereby causing 

 large subsidences in the drift. Some of these sub- 

 sidences are in steps (see plan and section, fig. 18, 

 19, PI. IV.)? as if tne ^ft when sinking had given 

 way successively along lines of parallel jointing, 

 thereby preventing the sand farthest away from the 

 vent having free egress. 



Remarkable instances of subsidence are those due 

 to a bed of sand being carried off by a spring, that 

 may come to the surface, at any distance away from 

 the source, of the sand. As the sand must be sub- 

 tracted particle by particle, the subsidence usually 

 is gradual, forming more or less bowl-shaped 

 hollows in the surface of the ground. This, however, 

 is not always the case, as the ground may give way 



