1840-1881] EARTH-MOVEMENTS 141 



case astronomical agencies should ever be proved or rendered Letter 492 

 probable, I imagine, as in nutation or precession, that an 

 upward movement or protrusion of fluidified matter below 

 might be immediately followed by movement of an opposite 

 nature. This is all that I meant. 



I have not read Jamieson, 1 or yet got the number. I was 

 very much struck with Forbes' 2 explanation of n[itrate] of 

 soda beds and the saliferous crust, which I saw and examined 

 at Iquique. I often speculated on the greater rise inland of 

 the Cordilleras, and could never satisfy myself. . . . 



I have not read Stur, 3 and am awfully behindhand in 

 many things. . . . 4 



To C. Lyell. Letter 493 



Down, July iSth [1867]. 

 The first part of this letter is published in Life and Letters, III. p. 71. 



Tahiti, 5 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. 6 

 Would not the Atlantic and Antarctic volcanoes be the best 



1 Possibly William Jameson, "Journey from Quito to Cayambe," 

 Geog, Soc. Journ., Vol. XXXI., p. 184, 1861. 



3 "On the Geology of Bolivia and Southern Peru," by D. Forbes, 

 Quart. Journ. Geol. Soc., Vol. XVII., p. 7, 1861. Mr. Forbes attributes 

 the formation of the saline deposits to lagoons of salt water, the com- 

 munication of which with the sea has been cut off by the rising of the 

 land (loc. cit., p. 13). 



3 Dionys Stur (1827-93), Director of the Austrian Geological Survey 

 from 1885 to 1892 ; author of many important memoirs on palasobotanical 

 subjects. 



4 The end of this letter is published as a footnote in Life and Letters, 

 II., p. 352. 



5 Tahiti (Society Islands) is coloured blue in the map showing the 

 distribution of the different kinds of reefs in The Structure and Distribu- 

 tion of Coral Reefs, Ed. in., 1889, p. 185. The blue colour indicates the 

 existence of barrier reefs and atolls which, on Darwin's theory, point 

 to subsidence. 



6 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 



