Geological Observations of Volcanic Islands | Page 7

interpretation. The views originally put forward by the old
German geologist and traveller, and almost universally accepted by his
countrymen, had met with much support from Elie de Beaumont and
Dufrenoy, the leaders of geological thought in France. They were,
however, stoutly opposed by Scrope and Lyell in this country, and by
Constant Prevost and Virlet on the other side of the channel. Darwin, in
the work before us, shows how little ground there is for the assumption
that the great ring-craters of the Atlantic islands have originated in
gigantic blisters of the earth's surface which, opening at the top, have
given origin to the craters. Admitting the influence of the injection of
lava into the structure of the volcanic cones, in increasing their bulk
and elevation, he shows that, in the main, the volcanoes are built up by
repeated ejections causing an accumulation of materials around the
vent.
While, however, agreeing on the whole with Scrope and Lyell, as to the
explosive origin of ordinary volcanic craters, Darwin clearly saw that,
in some cases, great craters might be formed or enlarged, by the
subsidence of the floors after eruptions. The importance of this agency,
to which too little attention has been directed by geologists, has
recently been shown by Professor Dana, in his admirable work on
Kilauea and the other great volcanoes of the Hawaiian Archipelago.
The effects of subsidence at a volcanic centre in producing a downward
dip of the strata around it, was first pointed out by Darwin, as the result
of his earliest work in the Cape de Verde Islands. Striking illustrations
of the same principle have since been pointed out by M. Robert and
others in Iceland, by Mr. Heaphy in New Zealand, and by myself in the
Western Isles of Scotland.
Darwin again and again called attention to the evidence that volcanic

vents exhibit relations to one another which can only be explained by
assuming the existence of lines of fissure in the earth's crust, along
which the lavas have made their way to the surface. But he, at the same
time, clearly saw that there was no evidence of the occurrence of great
deluges of lava along such fissures; he showed how the most
remarkable plateaux, composed of successive lava sheets, might be
built up by repeated and moderate ejections from numerous isolated
vents; and he expressly insists upon the rapidity with which the
cinder-cones around the orifices of ejection and the evidences of
successive outflows of lava would be obliterated by denudation.
One of the most striking parts of the book is that in which he deals with
the effects of denudation in producing "basal wrecks" or worn down
stumps of volcanoes. He was enabled to examine a series of cases in
which could be traced every gradation, from perfect volcanic cones
down to the solidified plugs which had consolidated in the vents from
which ejections had taken place. Darwin's observations on these points
have been of the greatest value and assistance to all who have essayed
to study the effects of volcanic action during earlier periods of the
earth's history. Like Lyell, he was firmly persuaded of the continuity of
geological history, and ever delighted in finding indications, in the
present order of nature, that the phenomena of the past could be
accounted for by means of causes which are still in operation. Lyell's
last work in the field was carried on about his home in Forfarshire, and
only a few months before his death he wrote to Darwin: "All the work
which I have done has confirmed me in the belief that the only
difference between Palaeozoic and recent volcanic rocks is no more
than we must allow for, by the enormous time to which the products of
the oldest volcanoes have been subjected to chemical changes."
Darwin was greatly impressed, as the result of his studies of volcanic
phenomena, followed by an examination of the great granite-masses of
the Andes, with the relations between the so-called Plutonic rocks and
those of undoubtedly volcanic origin. It was indeed a fortunate
circumstance, that after studying some excellent examples of recent
volcanic rocks, he proceeded to examine in South America many fine
illustrations of the older igneous rock-masses, and especially of the

most highly crystalline types of the same, and then on his way home
had opportunities of reviving the impression made upon him by the
fresh and unaltered volcanic rocks. Some of the general considerations
suggested by these observations were discussed in a paper read by him
before the Geological Society, on March 7th, 1838, under the title "On
the Connection of Certain Volcanic Phenomena, and On the Formation
of Mountain-chains, and the Effect of Continental Elevations." The
exact bearing of these two classes of facts upon one another are more
fully discussed