124 HOW CROPS FEED. 
many rocks consist of crystalline grains of distinct min- 
erals more or less intimately blended, is a point of weak- 
ness in their structure. ‘The grains of’ quartz, feldspar, 
and mica, of a granite, when exposed to changes of tem- 
perature, must tend to separate from each other; because 
the extent to which they expand and contract by alterna- 
tions of heat and cold are not absolutely equal, and be- 
cause, as Senarmont has proved, the same crystal expands 
or contracts unequally in its different diameters. 
Action of Freezing Water.—lIt 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 ;' 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 results, 
The adventurous explorer in the higher Swiss Alps fre- 
quently sees or hears the fall of fragments of rock 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.—Movine Water or Icr. 
Changes of temperature not only have created differ- 
ences of level in the earth’s surface, but they cause a con- 
