18401881] EARTH-MOVEMENTS 133 



subordinate part in the upward and downward movements of Letter 487 

 the earth's crust ; though of course it must swell the strata 

 where first affected. I can understand Sir J. Herschel's 

 manner of bringing heat to unheated strata namely, by 

 covering them up by a mile or so of new strata, and then 

 the heat would travel into the lower ones. But who can tell 

 what effect this mile or two of new sedimentary strata would 

 have from mere gravity on the level of the supporting 

 surface ? Of course such considerations do not render less 

 true that the expansion of the strata by heat would have some 

 effect on the level of the surface ; but they show us how 

 awfully complicated the phenomenon is. All young geologists 

 have a great turn for speculation ; I have burned my fingers 

 pretty sharply in that way, and am now perhaps become over- 

 cautious ; and feel inclined to cavil at speculation when the 

 direct and immediate effect of a cause in question cannot be 

 shown. How neatly you draw your diagrams ; I wish you 

 would turn your attention to real sections of the earth's 

 crust, and then speculate to your heart's content on them ; 

 I can have no doubt that speculative men, with a curb on, 

 make far the best observers. I sincerely wish I could have 

 made any remarks of more interest to you, and more directly 

 bearing on your paper ; but the subject strikes me as too 

 difficult and complicated. With every good wish that you 

 may go on with your geological studies, speculations, and 

 especially observations .... 



To C. Lyell. Letter 488 



Down, March 24th [1853]. 



I have often puzzled over Dana's case, in itself and in 

 relation to the trains of S. American volcanoes of different 

 heights in action at the same time (p. 605, Vol. V., Geolog. 

 Trans. 1 }. I can throw no light on the subject. I presume 

 you remember that Hopkins 2 in someone (I forget which) 



1 " On the Connexion of certain Volcanic Phenomena in South 

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

 the Effect of the same Power by which Continents are Elevated " 

 (Trans. GeoL Soc., Vol. V., p. 601, 1840). On p. 605 Darwin records 

 instances of the simultaneous activity after an earthquake of several 

 volcanoes in the Cordillera. 



2 See " Report on the Geological Theories of Elevation and Earth- 

 quakes," by W. Hopkins, Brit. Assoc. Rep., 1847, P- 34- 



