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