12 SHRINKAGE FISSURES RECENT DEPOSITS. 



tioned have taken place on mountain sides when 

 cracks have opened across the slopes, as in these 

 water will collect, till eventually large masses float, 

 and afterwards loosen and slide down into the 

 valleys. A slip of this kind took place on the 

 mountains of Glancastle, near Belmullet, County 

 "Mayo, in 1867, and a description of it was written by 

 the Rev. P. Mahon, P.P., of which the following is an 

 epitome : About forty acres of country, varying from 

 ten to fifteen feet in depth, broke loose from the 

 mountain-side and slid down into the Atlantic, 

 converting what had been a luxuriant dale into a 

 desolate valley. The slip was due to " the great 

 drought of the preceding two months creating a 

 vacancy between the peat and its gravelly substratum. 

 This was filled by the heavy fall of rain on July 19th, 

 by which the peat-covering of an entire mountain- 

 side was raised from its bed, broken up into huge frag- 

 ments, and caused to slide down the inclined surface, 

 carrying destruction in its course, until it spent its 

 fury in the Atlantic. The slope of the ground was 

 about one foot in fifteen." In this case it was a peaty 

 soil that gave way ; but similar slips may also occur 

 in drift, and they will hereafter be referred to when 

 we are describing bars and lake-basins in mountain 

 valleys. 



In alluvium (sand, gravel, and silt) cracks are not 

 so effective in this way as in peats, since the sides 



