INTRODUCTION 25 



or reducing them into dolomite ; or finally by inducing within 

 them the formation of crystals of the most varied composition. 

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

 cones of basalt, by the elastic forces emanating from the fluid 

 interior of our globe, has led one of the first geologists of the 

 age, Leopold von Buch, to the theory of the elevation o* 

 continents, and of mountain chains generally. This action of 

 subterranean forces in breaking through, and elevating strata 

 of sedimentary rocks, of which the coast of Chili, in conse- 

 quence of a great earthquake, furnished a recent example, 

 leads to the assumption, that the pelagic shells found by 

 M. Bonpland and myself on the ridge of the Andes, at an 

 elevation of more than 15,000 English feet, may have been 

 conveyed to so extraordinary a position, not by a rising of the 

 ocean, but by the agency of volcanic forces capable of ele- 

 vating into ridges the softened crust of the earth. 



I apply the term volcanic, in the widest sense of the word, 

 to every action exercised by the interior of a planet on its 

 external crust. The surface of our globe, and that of the moon, 

 manifest traces of this action, which in the former, at least, has 

 varied during the course of ages. Those, who are ignorant of 

 the fact, that the internal heat of the earth increases so ra- 

 pidly with the increase of depth, that granite is in a state of 

 fusion about twenty or thirty geographical miles below the 

 surface,* cannot have a clear conception of the causes, and 

 the simultaneous occurrence of volcanic eruptions at places 

 widely removed from one another, or of the extent and inter- 

 section of circles of commotion in earthquakes, or of the 

 uniformity of temperature, and equality of chemical com- 

 position observed in thermal springs during a long course of 

 years. The quantity of heat peculiar to a planet is, however, 

 a matter of such importance, being the result of its primitive 

 condensation, and varying according to the nature and 

 duration of the radiation, that the study of this subject may 



* The determinations usually given of the point of fusion are in 

 general much too high for refracting substances. According to the very 

 accurate researches of Mitscherlich, the melting point of granite can 

 hardly exceed 2372 F. 



[Dr. Mantell states in The Wtmder* of Geology, 1848, vol. i. page 34. 

 that this increase of temperature amounts to 1 of Fahrenheit .for ever) 

 *4 feet of vertical depth.] JV. 



