852 STUDIES FOR STUDENTS 



a powerful erosive action, particularly upon particles of rock of 

 such size as to be lifted or moved by wave action, but too heavy 

 to be protected from attrition by the thin film of water above 

 alluded to. Shaler's observation^ at Cape Ann were to the 

 effect that ordinary granitic paving blocks (weighing perhaps 

 twenty pounds) were, when exposed to surf action worn in the 

 course of a year into spheroidal forms such as to indicate an 

 average loss of more than an inch from their peripheries. Experi- 

 ments made with fragments of hard-burned brick showed that in 

 the course of a year they would be reduced fullv one-half their 

 bulk. Even the crystallization of the salt thrown up by wave 

 action and absorbed into the pores of rocks'" serves in its way 

 the purposes of disintegration. 



The action of freezing water and of ice. — ^The action of dry 

 heat and cold in disintegrating rocks has already been described. 

 The effects of such temperature changes upon stone of ordinary 

 dryness are however slight in comparison with the destructive 

 agencies of freezing temperatures upon stones saturated with 

 moisture. The expansive force of water passing from the liquid 

 to the solid state has been graphically described as equal to the 

 weight of a column of ice a mile high (about 150 tons to the 

 square foot). Otherwise expressed, one hundred volumes of 

 water expand, on freezing, to form one hundred and nine volumes 

 of ice. Provided then sufficient water be contained within the 

 pores of a stone, it is easy to understand that the results of 

 freezing must be disastrous. That stones as they lie in the 

 ground do contain moisture, often in no inconsiderable amounts, 

 is a fact well-known and well-recognized by all those engaged in 

 quarrying operations, and indeed no mineral substance is abso- 

 lutely impervious to it. The amount contained naturally varies 

 with the nature of the mineral constituents and their state of 

 aggregation. According to various authorities granite may con- 



'Bull. Geol. Soc. of America, Vol. V, p. 208. 



^According to Dana (Wilkes Exploring Expedition. Geology, p. 529) the 

 sandstones along the coast of Sydney, Australia, are subjected to a mechanical 

 disintegration through the crystallization of the salt which is absorbed from the saline 

 spray of the ocean waves. 



