180 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
Speaking in general terms, therefore, one may say that the 
demolition of rocks and the removal of their débris proceed most 
rapidly on steep slopes and along the natural drainage-lines of 
a country. On flat and gently-undulating ground the products 
of disintegration move so slowly that they tend to accumulate 
upon, or in the immediate vicinity of the rocks from which they 
come. On steeper declivities the rate of movement augments, 
while it attains a maximum in stream and river-courses. Every- 
where, therefore, from the tops of our highest mountains, down 
to the sea-coast, disintegrated rock-material is moving, slowly or 
more rapidly, as the case may be. 
Hitherto we have been considering the mechanical action of 
the superficial agents in the breaking-up of rocks and the 
removal of the materials. We may now glance at the chemical 
action which affects rocks, and greatly aids the mechanical 
action of the several agents referred to. Without going into 
detail, it may be said that rocks fall a more or less easy prey to 
the destructive energy of chemical changes, according as they 
consist of relatively soluble or insoluble ingredients. Rain-water 
always contains some proportion of carbon-dioxide and oxygen, 
derived partly from the atmosphere and partly from the 
vegetable soil through which it percolates. Charged with these 
re-agents, it is enabled to attack rocks of all kinds. Some of 
these, such as limestone, consist of materials which are readily 
soluble, and may thus be almost wholly removed in solution as 
bi-carbonates—only a meagre proportion of relatively insoluble 
matter being left behind. Other rocks again, such as quartzite, 
are composed of mineral matter which strongly resists the solvent 
action of rain, and owe their disintegration, therefore, almost 
entirely to the mechanical action of the superficial agents. 
Between these two extremes comes a very large series of rocks 
which are affected in various degrees by rain. Take granite as 
anexample. ‘This rock is composed of three minerals, namely, 
insoluble ! quartz, and two silicates known as felspar and mica. 
1 Probably there is no mineral matter quite unaffected by water. Under 
sufficient pressure, and at a high temperature (conditions which must obtain at 
no very great depth in the earth’s crust), even the most refractory minerals are 
attacked by water. But at the surface and at slight depths such a mineral as 
quartz is practically insoluble, while many other minerals are only dissolved 
with extreme difficulty. Many of these, however, although not directly 
soluble, are broken up chemically by rain-water, and transformed into soluble 
and relatively insoluble materials. 
