316 NEW ZEALAND NATURE-STUDY BOOK 
of a single rock. Suppose, as in Granite, the Felspar con- 
sists of a double silicate of Alumina and Potash (7.¢., a com- 
pound of Potash and Silica, and Alumina and Silica). The 
carbonated rain-water has the power of breaking down the 
link which binds the two compounds together, and of 
decomposing one of them—the Silicate of Potash. This is 
converted into Carbonate of Potash, which, being soluble, 
is quickly carried away in solution. Much of the binding 
material uniting the particles is in this way removed, so 
that the rock becomes loose and crumbling, admitting air 
and water, which continue the process of disintegration. 
The double silicates occurring in basalt, andesite, and 
other igneous rocks are acted on in a similar way. It is 
unnecessary to enter into further details, suffice that there 
are but few minerals which are not attacked by water con- 
taining impurities (Carbon-dioxide, Oxygen, and organic 
acids), and decomposed into simpler compounds, some of 
which are soluble. 
Mechanical action. We have already seen how 
frozen water or ice acts on rock masses—running water has 
the effect of removing the weathered crust and so exposing 
additional surface to further decomposition. This it does 
by washing away the loosened particles, the heavier the 
rainfall, the greater being the effect produced. 
And not only are the disintegrated particles of rock 
removed —the soil itself is continually being washed away 
and carried into the streams. The lower limit of the soil 
in this way gradually travels down into the sub-soil, which 
in turn encroaches further on the underlying rock. The 
net result of all this is that the upper portions of the 
soil are slowly but surely being carried to the sea, and the 
soil itself, which would otherwise become exhausted, is 
constantly replenished by additions from the underlying 
sub-soil and rock. 
