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.
B D K L C p H q m' o' m o “Whether Volcanos are in areas of
Extract of a Letter from C Lyell to C Darwin July 1. 1856—
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. 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 Atlantic 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! 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. A gradual sinking down of 6000 feet takes place in a short part of the
Miocene Period, (not occupying possibly above
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
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. 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.
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.
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.
- f1 1915a.f1The letter has not been found. The heading, date, and text given here are taken from Lyell's scientific journal 2, pp. 82–90 (Kinnordy House MS). It is also printed in Wilson ed. 1970, pp. 110–14.
- f2 1915a.f2Coral reefs (1842), pp. 140–2, ‘On the absence of active Volcanos in the areas of subsidence, and on their frequent presence in the areas of elevation’. Lyell had queried CD's suggestion that volcanoes were mostly associated with rising land in his scientific journal 2, p. 74, in an entry dated 30 June 1856 (Wilson ed. 1970, p. 108):
There has always seemed to me a difficulty in reconciling two facts in Darwin's theory of volcanic & Coral areas—namely that Volcanoes are the upheaving power and yet, that nearly all the islands in the middle of great oceans are volcanic, whereas there are not many active, nor an extraordinary number of Tertiary volcanoes in continental areas.
- f3 1915a.f3Maury 1855a, which Lyell had been studying with reference to the possibility of former land-bridges between Madeira and Africa (see Wilson ed. 1970, pp. 109–10).
- f4 1915a.f4Lyell had previously noted the correct depth of 1200 feet in his scientific journal 2, p. 80 (Wilson ed. 1970, p. 110).
- f5 1915a.f5Lyell refers to the first diagram given at the beginning of his extract, dated ‘June 1st
’, in his scientific journal (Wilson ed. 1970, p. 111).
- f6 1915a.f6Lyell refers to the second diagram given at the beginning of his extract.
- f7 1915a.f7C. Lyell 1830–3, 2: 283–301.
- f8 1915a.f8Following the letter, Lyell added:
P.S. not sent to Darwin The deepness of the sea round Madeira & Po So & other Atlantic Islands is against Volcanos being connected with upheaval for the upraising power wd tend at least to render the sea shallow shd it fail to push up dry land in the neighbourhood of oceanic volcanos. July 2d—/56