14 SHRINKAGE FISSURES RECENT DEPOSITS. 



bination of the fissures, but also because tlie pf eject- 

 ing angles of the intervening masses are liable to 

 break off or crumble away (fig. 5, PI. I.) Minor 

 joints may act similarly, and they are largely instru- 

 mental in causing reclaimed bog, unless well weighted 

 with clay or gravel, to return to its original condi- 

 tion. The process is as follows : After the bog is 

 laid down in grass, it cracks and opens during hot 

 weather. The fissures do not afterwards close, but 

 their sides shed, forming muddy spaces round patches 

 of grass. Yearly this opening goes on, until eventu- 

 ally the grass land turns into innumerable small 

 heathery hummocks with soft bog between, moss 

 and heather having killed out the grass. A similar 

 process may occur in alluvial flats, except that rushes, 

 instead of moss and heather, usually succeed the 

 grass. 



In any recent accumulation, one or more layers 

 may be composed of sand, marl, or some such material 

 that will "run" when a section is exposed, either 

 naturally or artificially. Fine sand and the like 

 usually runs, on account of the water it contains, 

 moving off and carrying the sand with it. Slips of 

 drift, due to this cause, may be studied on the coast 

 of Counties Mayo and Galway, in places where there 

 are accumulations of boulder- clay drift containing 

 lenticular layers and subordinate beds of sand. Some 

 of the latter, when exposed by the sea action, " run ; " 



