THE FIELD NATURALIST'S KNOWLEDGE.
55
chemical action. Every field naturalist knows that to the peroxida-
tion of iron is due the reds and browns, the buffs and the yellows
of our sands, clays and hard rocks, and notably of the surface Lon-
don Clay of the south-east of England, that the green grains of the
Greensands owe their greenness to silicate of iron, that the blues and
greens of some of our Triassic rocks are produced by salts of copper,
and that the dark colours of carbonaceous shales, lignites, and coal,
are the consequence of the elimination of oxygen and nitrogen from
the organic materials of those beds.
The formation, too, of the rocks themselves is largely due to
chemical action, as is likewise their destruction. This is especially
the case with respect to the limestones. The precipitation of the
material of hard fresh-water limestone has continued on a large scale
in Italy, where the carbonated rain-water has dissolved the limestone
of the Apennines, and, losing much of its solvent power by the loss
of its carbonic acid, it re-deposits carbonate of lime, and so forms
the massive travertine that supplied the builders of Old Rome with
materials for their world-renowned edifices.
Analogous to the formation of travertine by the precipitation of
carbonate of lime, is the production of silicious sinter by geysers and
boiling springs. Hot water has a power of dissolving more silica
than it can hold when cold, and so on cooling solid silica is deposited.
Thus is explained the formation not only of the silicious mounds
around the geysers of Iceland and those greater ones of the Yellow-
stone Park in America, but also of those marvellously beautiful
pink and white terraces of New Zealand, the destruction of which
by a volcanic eruption a few years ago occasioned so much regret
and deprived the Great Britain of the southern hemisphere of a most
wonderful as well as most attractive feature.
The deep gorges of our south country chalk downs, the perpen-
dicular precipices of our Derbyshire river valleys and of Cheddar
cliffs, and too of the immensely greater limestone precipices of other
lands, are mainly due to the solvent action of carbonated surface-
water. So, likewise, are the caverns of Derbyshire, and of that place
beloved of Palaeozoic geologists and Trilobite-hunting palaeontolo-
gists, Dudley, as well as the famous Blue Grotto of Capri, and the
caves of other limestone districts. And in the softer rocks of the
south of England, apart from the Chalk, chemical action is by no
means inconspicuous. For in our Lower Greensand we have hard
beds and hard concretionary masses produced by the cementation