124 GEOLOGY [CHAP. IX 



Letter 483 easily explain the S. Wales and Eocene cases. I will only 

 add that I should think there would be a little more 

 sediment produced during subsidence than during elevation, 

 from the resulting outline of coast, after long period of rise. 

 There are many points in my volume which I should like to 

 have discussed with you, but I will not plague you : I should 

 like to hear whether you think there is anything in my 

 conjecture on Craters of Elevation 1 ; I cannot possibly believe 

 that Saint Jago or Mauritius are the basal fragments of 

 ordinary volcanoes ; I would sooner even admit E. de Beau- 

 mont's views than that much as I would sooner in my 

 own mind in all cases follow you. Just look at p. 232 2 in 

 my 5\ America for a trifling point, which, however, I 

 remember to this day relieved my mind of a considerable 

 difficulty. I remember being struck with your discussion on 

 the Mississippi beds in relation to Pampas, but I should 



1 In the Geological Observations on Volcanic Islands, 1844, pp. 93-6, 

 Darwin speaks of St. Helena, St. Jago and Mauritius as being bounded 

 by a ring of basaltic mountains which he regards as " Craters of Eleva- 

 tion." While unable to accept the theory of Elie de Beaumont and 

 attribute their formation to a dome-shaped elevation and consequent 

 arching of the strata, he recognises a " very great difficulty in admitting 

 that these basaltic mountains are merely the basal fragments of great 

 volcanoes, of which the summits have been either blown off, or, more 

 probably, swallowed by subsidence." An explanation of the origin and 

 structure of these volcanic islands is suggested which would keep them 

 in 'the class of " Craters of Elevation," but which assumes a slow 

 elevation, during which the central hollow or platform having been 

 formed " not by the arching of the surface, but simply by that part 

 having been upraised to a less height." 



2 This probably refers to a paragraph (p. 232) " On the Eruptive 

 Sources of the Porphyritic Claystone and Greenstone Lavas." The 

 opinion is put forward that "the difficulty of tracing the streams of 

 porphyries to their ancient and doubtless numerous eruptive sources, 

 may be partly explained by the very general disturbance which the 

 Cordillera in most parts has suffered"; but, Darwin adds, "a more 

 specific cause may be that 'the original points of eruption tend to 

 become the points of injection.' . . . On this view of there being a 

 tendency in the old points of eruption to become the points of sub- 

 sequent injection and disturbance, and consequently of denudation, it 

 ceases to be surprising that the streams of lava in the porphyritic 

 claystone conglomerate formation, and in other analogous cases, should 

 most rarely be traceable to their actual sources." The latter part of 

 this letter is published in Life and Letters, I., pp. 377, 378. 



