v.] THE WORK OF FROST 8i 



lating water, then, acts both by pure solution and by 

 starting more drastic chemical changes by the help of 

 the carbon dioxide it carries ; its work is often assisted 

 by the penetration of roots into the cracks and fissures. 

 The roots, both of the larger forms of vegetation and of 

 trees, will travel for astonishing distances and will 

 squeeze themselves in the pursuit of water into the 

 closest of cracks ; as they grow they exert considerable 

 pressure to widen the cracks, and when they die and 

 decay they leave behind a channel for percolation. 



The main agency, however, in splitting rocks and 

 reducing them to the state of soil is frost, or rather 

 water in the act of freezing. It is well known that 

 water expands about one-tenth of its bulk in freezing, 

 and that it will exert enormous pressure on anything 

 that tends to prevent such solidification and expansion. 

 Hence the bursting of water-pipes, full bottles, and the 

 like during a severe frost ; though, as the bursting only 

 becomes apparent when the water thaws again, it is often 

 supposed that the thaw does the bursting. Hence also 

 the constant fracture of stones or bricks exposed to the 

 weather during winter; they become saturated with 

 water, and when this water tries to expand within them 

 they must split to relieve the pressure, thus developing 

 whatever lines of weakness there were within. The 

 examination of the face of an old quarry or even an old 

 wall of brick or stone after a frost will show the ground 

 at the foot of the face strewn with blocks and fragments 

 which have been wedged off by the expansion of the 

 water within the stone or brick. We talk of the 

 pulverising action of frost on the clods of a stiff clay soil ; 

 frost is equally if less actively pulverising to the toughest 

 and most closely grained of rocks. There are other 

 minor agencies assisting, but in the main, water, roots, 

 and frost are the tools which are always at work reducing 



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