tening flakes of
mica which plainly are derived from the original granite. Washing
off the finer particles, we find the largest remaining grains are
composed of the all but indestructible quartz.

[1] Proc. Roy. Irish Acad., VIII., Ser. A, p. 21.

37

This also is from the granite. Some few of the grains are of
chalky-looking felspar; again a granitic mineral. What is the
finer silt we have washed off? It, too, is composed of mineral
particles to a great extent; rock dust stained with iron oxide
and intermixed with organic remains, both animal and vegetable.
But if we make a chemical analysis of the finer silt we find that
the composition is by no means that of the granite beneath. The
chemist is able to say, from a study of his results, that there
has been, in the first place, a large loss of material attending
the conversion of the granite to the soil. He finds a
concentration of certain of the more resistant substances of the
granite arising from the loss of the less resistant. Thus the
percentage amount of alumina is increased. The percentage of iron
is also increased. But silica and most other substances show a
diminished percentage. Notably lime has nearly disappeared. Soda
is much reduced; so is magnesia. Potash is not so completely
abstracted. Finally, owing to hydration, there is much more
combined water in the soil than in the rock. This is a typical
result for rocks of this kind.

Deeper in the soil we often observe a change of texture. It has
become finer, and at the same time the clay is paler in colour.
This subsoil represents the finer particles carried by rain from
above. The change of colour is due to the state of the iron which
is less oxidised low down in the soil. Beneath the subsoil the
soil grows

38

again coarser. Finally, we recognise in it fragments of granite
which ever grow larger as we descend, till the soil has become
replaced by the loose and shattered rock. Beneath this the only
sign of weathering apparent in the rock is the rusty hue imparted
by the oxidised iron which the percolating rain has leached from
iron-bearing minerals.

The soil we have examined has plainly been derived in situ from
the underlying rock. It represents the more insoluble residue
after water and acids have done their work. Each year there must
be a very slow sinking of the surface, but the ablation is
infinitesimal.

The depth of such a soil may be considerable. The total surface
exposed by the coun