PEECHED BLOCKS AND ASSOCIATED PHENOMENA. 



533 



So when we are examining, not the cliffs but the tops of the hills, 

 we find that the mode of weathering of the limestone is influenced 

 by the same differences of character, and the result is an endless 

 variety of fantastic forms in one case, and a more or less uniform 

 crumbling away in another. 



Of external circumstances affecting the mode of weathering of the 

 limestone the most important is the vegetation. We see crevices, 

 where there can never be any mechanical abrasion' of the rock, 

 opened out into great chasms as lichen, moss, ferns, and grass succes- 

 sively get foothold in it ; and round the margin of the great lime- 



Eig. 4. — Ground-plan shoiving Mode of Weathering of Mountain ^ 

 Limestone. (1 inch=2 feet 8 inches.) 





^ 1^ 



stone tables on which the boulders we are describing rest we see 

 the manner in which the rock is eaten back as the vegetation 

 encroaches along the lines of weakness, furnishing more acid and 

 holding more damp (see fig. 4). 



Q. J. G. S. No. 168. 2 o 



