§ 2. NATURE OF VOLCANIC ACTION. 807 



internal heat of our planet on its external surface, during 

 its different states of refrigeration and condensation : by 

 which concussions of the land, or earthquakes, and the 

 expansion or elevation, and the subsidence or contraction, 

 of large portions of the solid crust of the earth, have been 

 produced. The number of existing volcanoes is estimated 

 at about 200, of which 116 are situated in America, or its 

 islands. 



In the previous discourses, many of the effects of igneous 

 agency came under our notice,* namely, the subsidence 

 and elevation of the Temple of Serapis (p. 106) ; the 

 gradual rise of Scandinavia (p. 116) ; the upheaving of the 

 sea-coast of Chili (p. 112) ; and other mutations of a like 

 nature. As we successively examined the tertiary, 

 secondary, and palaeozoic formations, proofs that similar 

 phenomena had taken place during every geological epoch, 

 were equally manifest ; the geographical distribution of 

 the foci of volcanic action was found to have varied, but 

 throughout the cycle of physical changes contemplated by 

 Geology, the volcano and the earthquake appear to have 

 been in ceaseless activity. The immediate cause of volcanic 

 action, though not demonstrable, seems to be connected 

 with the fact of a constantly increasing temperature in the 

 interior of the earth, according to the distance from its 

 surface ; and which internal heat is probably induced by 

 the excitement of electro -magnetic currents on the mineral 

 matter which composes the mass of our planet. This 

 high temperature of the interior, whatever may be its 

 origin, is the germ not only of earthquakes, which are the 

 purely dynamical effects of volcanic action, but also of 

 the gradual elevation of continents, and of chains of moun- 

 tains from extended fissures ; giving rise to eruptions of 

 lava, mud, boiling water, &c. ; to thermal mineral springs, 



* A concise view of the effects of high temperature and of volcanic 

 action on the earth's crust, was given in the first Lecture, ante, p. 103. 



