PHYSICAL KNOWLEDGE. 27 



introduction of silex, or changing them into dolomite, or 

 causing in them the formation of crystals of various kinds. 

 The elevation of sporadic islets, of domes of trachyte, and 

 of cones of basalt, by the elastic forces which emanate from 

 the fluid interior of our planet, conducted the first geologist 

 of our age, Leopold von Buch, to the theory of the elevation 

 of continents and of mountain chains generally. This ac- 

 tion of subterranean forces, in breaking through and elevat- 

 ing sedimentary rocks, of which the coast of Chili has 

 offered a recent example, shows us how the oceanic shells 

 which M. Bonpland and myself found on the ridge of the 

 Andes, at an elevation of more than 4600 metres (about 

 15100 English feet), may have been conveyed there, not 

 by a rise of the ocean, but by volcanic agencies elevating 

 into ridges the heat-softened crust of the globe. 



I use the term volcanic agency in its most general sense, 

 applying it, whether on the earth or on her satellite the 

 moon, to that reaction of the interior of a planet on its 

 crust, which on our globe at least has been very different 

 at different epochs. Those who are unacquainted with the 

 experiments which show the increase of internal heat at 

 increasing depths in the earth to be so rapid, that granite ( 13 ) 

 is supposed to be in a state of fusion about twenty geogra- 

 phical miles below the surface, cannot have a clear compre- 

 hension of the causes of the simultaneity of volcanic eruptions 

 occurring at great distances apart, of the extent and in- 

 tersection of circles of commotion in earthquakes, of 

 the constancy of temperature and of chemical composition 

 in thermal springs during many years of observation, or 

 of the difference of temperature of Artesian wells of unequal 

 depth, And yet the knowledge of the internal terrestrial 



