From Charles Lyell [1 July 1856]
[DIAGRAM HERE] A F G B D K L C p H q m’ o’ m o “Whether Volcanos are in areas of elevation” ramme
Extract of a Letter from C Lyell to C Darwin July 1. 1856—1
Of course it is true, as you well show in your Coral volume, that the active volcanos have recent deposits with marine shells uplifted in them.2 This is the case to some small extent in all the principal Atlantic islands except Palma, which I visited, & Palma has not been thoroughly examined & may somewhere exhibit signs of elevation Comparatively therefore, and by contrast with the Atoll areas, you may represent the volcanic as rising.
Still I have always felt a little uncomfortable at being called upon to assume that in recent & pliocene ages volcanic action has been and is connected with the growth of land. Were this the case should we not find that the continents would be the great areas of extinct Pliocene and of active volcanos, and that the latter did not affect sea-side and insular and even mid-ocean sites.
If we find active volcanos in Oceanic areas, & few or none of them in the middle of continental areas, it furnishes a primâ facie case in favour of the doctrine that the grand uplifting power acts very independently of the accidental sites of existing superficial outbreaks. An argument might even be raised in support of the theory that active volcanos are more connected with sinking on a great scale however true it may be that locally they tend to upheave as well as to form land by outpouring of lava & of ejectamenta.
Maurys last chart of the Atlantic3 makes the Atlantis hypothesis more bold than it appeared when E. Forbes proposed it for the Canaries are separated from Africa & Europe by deep sea depressions of more than 6000 feet & Madeira by depths exceeding 12,000 feet!4 The data, I fear are scanty however.
I find in Madeira & the Canaries upraised littoral deposits of the Miocene period, in my sense of Miocene when there was a certain proportion of living species already in being. This, I think, rather increases the difficulty of the continental extension hypothesis.
But I want to ask you whether it may not be true that the bed of the Atlantic has been gradually sinking all the while the Canaries & Madeiras have been forming & that very slight local upheaval only has occurred even on the sites of these volcanic islands. I sometimes think I can dispense with all excess even of local upheaval, over & above that of the adjoining deep sea spaces.
Thus for example suppose A. B. C. to represent the original Europeo-African continent & B. D. to be the level of the Atlantic.5 A gradual sinking down of 6000 feet takes place in a short part of the Miocene Period, (not occupying possibly above a million of years.
The ocean has thus risen relatively to the land up to G. But in the meantime the volcano F. has been gradually built up 7500 ft & is 1500 feet higher above the sea. A pause in the volcanic action takes place during which a subsidence of partial extent under Id. occurs causing F. to lose 1500 feet of its height by slow depression during which every part of the subaerial mass of F. gets submerged & covered or faced with a marine littoral deposit, full of rolled boulders and pebbles with
[DIAGRAM HERE] q H p
patellæ, & other littoral shells. The subjacent rocks H. all volcanic but as entirely free from marine remains as if exclusively subaerial.
We now have the original subterranean layer K. L. bending down at m, n, o. 1500 feet below the general depression of 6000 feet, & if it be then restored to the level m’ o’ we have the volcano H. pushed up again 1500 feet with the marine beds p. q., abutting against the foundation of older subaerial rocks.6 This is what I observed in one part of the Grand Canary.
The 4000 or 5000 of additional subaerial volcanic beds may be built up & you have Madeira. In the Grand Canary I suspect most of its height was attained before the submergence of 1500 or (1100? feet. [DIAGRAM HERE] marine beds older subaerial Grand Canary
But my reasoning you see is the same as that which I adopted about the Atolls before you invented your theory, namely, that oscillations occurring in a sea filling up with coral or with volcanic matter may cause uplifted marine formations provided subsidence & upheaval be just equal the one to the other.7
Take away all the volcanic matter from Etna, Ischia &c & the marine shells could sink down below the sea level. All the marine beds in the Canaries & Madeiras are volcanic except the corals & shells themselves. If the active volcanos were connected with a continent-making power, we should see secondary and non-volcanic rocks uplifted by them.
I do not however want to contend that active & Pliocene volcanos belong to subsiding areas rather than to areas of elevation altho’ half inclined to that alternative in preference to the opposite theory. But surely they are so distributed as that they seem to belong quite as much to Pliocene & recent subsidence as to upheaval during the same period.8
To cast doubt on CD’s view that volcanic action is associated with elevation of land, CL suggests that local oscillations in strata underlying volcanoes could also explain how active volcanoes have uplifted fossil deposits of marine shells. Overall he is more inclined to believe that recent volcanoes belong to areas of subsidence rather than of elevation.
Please cite as
Darwin Correspondence Project, “Letter no. 1915A,” accessed on 20 January 2017, http://www.darwinproject.ac.uk/DCP-LETT-1915A