SOIL: ITS ORIGIN AND NATURE. 179 
how water which has found its way into the fissures of a rock 
tends to widen these when it freezes. Rock-surfaces exposed 
to such action naturally become fractured and shattered, as may 
be seen everywhere amongst our mountains, the slopes of which 
are so frequently mantled by frost-riven débris. When water 
freezes in the minute pores of rocks, it tends in like manner to 
force the grains and particles of the rock asunder, so that when 
thaw ensues the disintegrated material is ready to be removed— 
in other words, the rock crumbles. Although frost is the most 
conspicuous rock-destroyer in this country, it is by no means the 
only agent of destruction. Rocks expand superficially under 
the heat of the sun, and contract again when cooling supervenes. 
Such alternate expansion and contraction loosens the cohesion 
of the mineral constituents of rocks, and thus so far plays the 
same part as frost. But this particular action, it is needless to 
Say, is most notable in regions where the diurnal range of 
temperature much exceeds that experienced in temperate 
latitudes. 
The mechanical action of rain is not less obvious than that 
of frost. It washes rock-surfaces and removes disintegrated 
materials, and the same operation is performed by water derived 
from melting snow. Wind, also, to some considerable extent, 
tends to sweep away the grit and dust derived from crumbling 
rock-masses. In this country, however, rain and running water 
are undoubtedly the most active agents employed in removing 
disintegrated rock-materials from higher to lower levels. Slowly, 
or more rapidly, as the case may be, these eventually find their 
way into the natural drainage-system of a country. Much of 
the sediment carried by streams and rivers consists of the dis- 
integrated rock-material washed by rain and melting snow from 
the surface of the land. Thus the mechanical action of rain 
cannot be considered without reference to that of streams and 
rivers. The latter, as we all know, are not mere carriers of 
sediment, but effective agents of erosion. By means of the 
sand, grit, and gravel which they sweep along, they grind the 
rocky surfaces over which they flow, and, at the same time, 
undermine their banks, and thus are ever deepening and widen- 
ing their courses. Much of the sediment resulting from this 
ceaseless process of erosion eventually escapes to sea, but not 
a little is caught in lakes or lodged within valleys, where it forms 
our well-known alluvial plains and terraces. 
