314 | NEW ZEALAND NATURE-STUDY BOOK 
and small, are broken off, thus exposing fresh surfaces to 
the action of weathering agents. Even if but few 
fragments are detached, the rock in time becomes friable 
and disintegrated, for, when the frozen water thaws, there - 
is nothing to force the displaced particles back into their 
former positions. 
Water. By far the most important weathering agent 
is water. We have already seen what a powerful factor it 
is in tearing asunder rock-masses at the moment of its 
conversion into ice. In other ways its action is even more 
noticeable—some of which will now be detailed. 
The condensed vapour of the atmosphere descends upon 
the land as rain, hail, or snow. The rain either sinks 
underground and appears again as springs, or pours down 
the land-slopes, feeding brooks and rivers. The snow, when 
it falls on higher elevations, gathers in great snow-fields, is 
pressed together and converted into ice, and moves down 
towards the plains and valleys in the form of glaciers. 
These, in the warmer atmosphere of the lower levels, 
gradually melt, and flow to the sea in turbid streams. 
Practically the whole of the water precipitated, is in this 
way returned, to the oceans. When it ascends into the air 
in the form of vapour, it is comparatively free from 
impurities, but, after being condensed, descending through 
the air, and making its way under the surface of the 
ground, it is no longer pure, but is laden with material 
collected on its journey. The presence of these impurities 
immensely increases its weathering properties. 
Water acts in two ways—chemically and mechani- 
cally. 
Chemical action. On its passage through the air, 
the rain absorbs several of the gases of which the air is 
composed—notably Oxygen and Carbon-dioxide. As soon 
as it reaches the ground, it begins to absorb other 
