EARTH'S SURFACE METEORIC ABRASION. 77 



fissures about six inches across, and many yards in 

 length and depth. When the rain falls, it finds its way 

 into these, and softens the clay at the bottom. If 

 the crack is near the cliff the half detached mass slips 

 down to the shore below." 1 Mr A. Geikie, F.R.S., in 

 his description of the formation of a valley or ravine, has 

 to acknowledge the necessity of the shrinkage fissures ; 

 for while writing of the action of springs, he states 

 that the water percolates through " the joints and 

 fissures of the rocks." Similarly, when describing 

 frost action, the necessary assistance of joints and 

 fissures is mentioned ; and in the description of 

 the denudation of a rock to form a cliff, there is the 

 following : " There is no long escarpment or cliff of 

 such materials which is not traversed with joints, 

 faults, or other divisional planes, that serve as 

 subterranean channels for the water." 2 So also has 

 our colleague, Mr J. C. Ward, when writing of the 

 scenery of the Lake District, England ; as he says of 

 the weathering of the hills over Derwentwater : 

 " Numerous irregular joints and fissures assist in the 

 work, and hence the craggy hill-sides." 



The sun opens cracks and other shrinkage fissures ; 

 and without these, as we have pointed out so often, 

 none of the denudants can work ; yet all breaks of 



1 "Mem. Geol. Survey." The Geology of the London Basin. Part 

 I., p. 387. 



2 "Scenery of Scotland," pp. 35-37. 



