1840-1881] l \ RTH-MCH | \< ENTS I.p 



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 18th [1867]. 

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



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 llcrschel on Volcanic Islands. 6 

 Would not the Atlantic and Antarctic volcanoes be the best 



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

 Geoje. Soc. Jour/i., Vol. XXXI., p. 1S4, 1861. 



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

 Quart- Journ. Geo/. Soc.,Vo\. XVII., p. 7, t86i. 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. eit., p. 13). 



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

 from 18S5 to 1892 ; author of many important memoirs on pakcobotanical 

 subjects. 



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

 II., p. 352. 



B 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 



