Sir John Herschel suggested that the accumulation on the sea-floor of sediment, derived from the waste of the island, presses down the bed of the ocean, the continent being on the other hand relieved of pressure; "this brings about a state of strain in the crust which will crack in its weakest spot, the heavy side going down, and the light side rising." In discussing this view Lyell writes ("Principles," Volume II. Edition X., page 229), "This hypothesis appears to me of very partial application, for active volcanoes, even such as are on the borders of continents, are rarely situated where great deltas have been forming, whether in Pliocene or post-Tertiary times. The number, also, of active volcanoes in oceanic islands is very great, not only in the Pacific, but equally in the Atlantic, where no load of coral matter...can cause a partial weighting and pressing down of a supposed flexible crust.") Would not the Atlantic and Antarctic volcanoes be the best examples for you, as there then can be no coral mud to depress the bottom? In my "Volcanic Islands," page 126, I just suggest that volcanoes may occur so frequently in the oceanic areas as the surface would be most likely to crack when first being elevated. I find one remark, page 128 (493/4. "Volcanic Islands," page 128: "The islands, moreover, of some of the small volcanic groups, which thus border continents, are placed in lines related to those along which the adjoining shores of the continents trend" [see Figure 5].), which seems to me worth consideration--viz. the parallelism of the lines of eruption in volcanic archipelagoes with the coast lines of the nearest continent, for this seems to indicate a mechanical rather than a chemical connection in both cases, i.e. the lines of disturbance and cracking. In my "South American Geology," page 185 (493/5. "Geological Observations on South America," London, 1846, page 185.), I allude to the remarkable absence at present of active volcanoes on the east side of the Cordillera in relation to the absence of the sea on this side. Yet I must own I have long felt a little sceptical on the proximity of water being the exciting cause. The one volcano in the interior of Asia is said, I think, to be near great lakes; but if lakes are so important, why are there not many other volcanoes within other continents? I have always felt rather inclined to look at the position of volcanoes on the borders of continents, as resulting from coast lines being the lines of separation between areas of elevation and subsidence. But it is useless in me troubling you with my old speculations.
LETTER 494. TO A.R. WALLACE. March 22nd [1869].
(494/1. The following extract from a letter to Mr. Wallace refers to his "Malay Archipelago," 1869.)
I have only one criticism of a general nature, and I am not sure that other geologists would agree with me. You repeatedly speak as if the pouring out of lava, etc., from volcanoes actually caused the subsidence of an adjoining area. I quite agree that areas undergoing opposite movements are somehow connected; but volcanic outbursts must, I think, be looked at as mere accidents in the swelling up of a great dome or surface of plutonic rocks, and there seems no more reason to conclude that such swelling or elevation in mass is the cause of the subsidence, than that the subsidence is the cause of the elevation, which latter view is indeed held by some geologists. I have regretted to find so little about the habits of the many animals which you have seen.
LETTER 495. TO C. LYELL. Down, May 20th, 1869.
I have been much pleased to hear that you have been looking at my S. American book (495/1. "Geological Observations on South America," London, 1846.), which I thought was as completely dead and gone as any pre-Cambrian fossil. You are right in supposing that my memory about American geology has grown very hazy. I remember, however, a paper on the Cordillera by D. Forbes (495/2. "Geology of Bolivia and South Peru," by Forbes, "Quart. Journ.