136 GEOLOGY [Chap. IX 



Letter 489 viz. maritime position of volcanoes ; and still more from the 

 immense thicknesses of Silurian, etc., volcanic strata, which 

 thicknesses at first impress the mind with the idea of sub- 

 sidence. If this could be proved, the theory would be 

 smashed ; but in deep oceans, though the bottom were rising, 

 great thicknesses of submarine lava might accumulate. But 

 I found, after writing Coral Book, cases in my notes of sub- 

 marine vesicular lava-streams in the upper masses of the 

 Cordillera, formed, as I believe, during subsidence, which 

 staggered me greatly. With respect to the maritime position 

 of volcanoes, I have long been coming to the conclusion that 

 there must be some law causing areas of elevation (con- 

 sequently of land) and of subsidence to be parallel (as if 

 balancing each other) and closely approximate ; I think this 

 from the form of continents with a deep ocean on one side, 

 from coral map, and especially from conversations with you 

 on immense subsidences of the Carboniferous and [other] 

 periods, and yet with continued great supply of sediment. 

 If this be so, such areas, with opposite movements, would 

 probably be separated by sets of parallel cracks, and would 

 be the seat of volcanoes and tilts, and consequently volcanoes 

 and mountains would be apt to be maritime ; but why 

 volcanoes should cling to the rising edge of the cracks I 

 cannot conjecture. That areas with extinct volcanic archi- 

 pelagoes may subside to any extent I do not doubt. 



Your view of the bottom of Atlantic long sinking with 

 continued volcanic outbursts and local elevations at Madeira, 

 Canaries, etc., grates (but of course I do not know how com- 

 plex the phenomena are which are thus explained) against 

 my judgment ; my general ideas strongly lead me to believe 

 in elevatory movements being widely extended. One ought, 

 I think, never to forget that when a volcano is in action we 

 have distinct proof of an action from within outwards. Nor 

 should we forget, as I believe follows from Hopkins, 1 and as 

 I have insisted in my Earthquake paper, 2 that volcanoes and 



1 " Researches in Physical Geology," W. Hopkins, Trans. Phil. Soc. 

 Cambridge, Vol. VI., 1838. See also " Report on the Geological Theories 

 of Elevation and Earthquakes," W. Hopkins, Brit. Assoc. Rep., p. 33, 

 1847 (Oxford meeting). 



2 "On the Connexion of certain Volcanic Phenomena in S. America, 

 and on the Formation of Mountain Chains and Volcanoes, as the Effect 



