LETTER 493. TO C. LYELL. Down, July 18th [1867].

(493/1. The first part of this letter is published in "Life and Letters," III., page 71.)

(493/2. Tahiti (Society Islands) is coloured blue in the map showing the distribution of the different kinds of reefs in "The Structure and Distribution of Coral Reefs," Edition III., 1889, page 185. The blue colour indicates the existence of barrier reefs and atolls which, on Darwin's theory, point to subsidence.)

Tahiti is, I believe, rightly coloured, for the reefs are so far from the land, and the ocean so deep, that there must have been subsidence, though not very recently. I looked carefully, and there is no evidence of recent elevation. I quite agree with you versus Herschel on Volcanic Islands. (493/3. 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,"