124 HOW mors feed. 



many rocks consist of crystalline grains of tlistinct niin 

 erals more or less intLmately blended, is a point of weak- 

 ness in their structure. The grains of quartz, feldspar, 

 and mica, of a granite, m lien exposed to changes of tem- 

 perature, must tend to separate from each other; because 

 the extent to which they expand and contiact by alterna- 

 tions of heat and cold are not absolutely equal, and be- 

 cause, as Senarmont has provdl, the same crystal expands 

 or contracts unequally in its different diameters. 



Action of Freezing Water. — It is, however, when wa- 

 ter insinuates itself into the slight or even imperceptible 

 rifts thus opened, and then freezes, that the process of dis- 

 integration becomes more rapid and more vigorous. Wa- 

 ter in the act of conversion into ice expands j\ of its bulk, 

 and the force thus exerted is sufficient to burst vessels of 

 the strongest materials. In cold latitudes or altitudes this 

 agency working through many years accomplishes stupen- 

 dous residts. 



The adventurous explorer in the higher Swiss Alps fre- 

 quently sees or hears the fall of fragments of i-ock thus 

 loosened from the peaks. 



Along the base of the vertical trap cliffs of New Haven 

 and the Hudson River, lie immense masses of broken rock 

 reaching to more than half the height of the bluffs them- 

 selves, rent off by this means. The same cause operates 

 in a less conspicuous but not less important way on the 

 surface of the stone, loosening the minute grains, as in 

 the above instances it rends off enormous blocks. A 

 smooth, clean pebble of the very compact Jura limestone, 

 of such kind, for example, as abound in the rivers of 

 South Bavaria, if moistened with water and exposed over 

 night to sharp frost, on thawing, is muddy with the de- 

 tached particles. 



2. — Moving Water or Ice. 



Changes of temperature not only have created differ- 

 ences of level in the earth's surface, but they cause a con- 



