

SOILS AND SUBSOILS 371 
parts of Europe and North America has been formed, for the 
most part, subsequent to the disappearance of glacial con- 
ditions; while in southern regions, rock decay has gone on 
without interruption ever since those lands came _ into 
existence. 
Action of Frost.—The disintegrating action of rain in 
temperate and high: latitudes is greatly aided by frost, and 
the same is the case in the elevated tracts of southern 
latitudes. Rain renders the superficial parts of rocks more 
porous, and thus enables frost to act more effectually ; while 
frost, by widening pores and fissures, affords readier ingress 
to meteoric water. Water freezing in soils and subsoils, and 
in the interstitial pores and minute fissures of rocks, forces 
the grains and particles asunder, and when thaw ensues, the 
loosened material is ready to be carried away by rain or 
melting snow, and subsequently, it may be, by wind. The 
same process takes place on a larger scale in the prising-open 
of joints and bedding-planes, and the consequent rending 
asunder of rocks. This action is best seen in Arctic regions 
and at high levels in our own and other countries, where the 
solid rocks not infrequently become buried underneath their 
own ruins. By and by, however, these loose, angular frag- 
ments are shattered, crumbled, and pulverised by frost, until 
they are in a condition to be swept away by wind, rain, or 
melting snow. The solid rock then comes again within reach 
of the same destructive action, and so the work of disruption 
and disintegration continues. 
Action of Plants and Animals.—The acids derived from 
decaying organic matter are powerful agents of chemical 
change. Without their aid, rain would be a much less effec- 
tive worker. Living plants themselves, however, attack 
rocks, and by means of the acids in their roots, dissolve out 
the mineral matter required by the organisms. Further, 
their roots penetrate the natural division-planes of rocks and 
wedge them asunder ; and thus, by allowing freer percolation 
of water they prepare the way for more rapid disintegration. 
Nor can we overlook the part played by tunnelling and 
burrowing animals, some of. which aid considerably in the 
work of reducing rocks. Worms, for example, by triturating 
in their gizzards the stony particles of a soil, reduce these in 
