ON ITS EXTERIOR. GENERAL REMARKS. 165 



view. We can imagine upheavings and formative acti- 

 vity in the production of crystalline rocks from a molten 

 mass, even in a body supposed to be destitute of air and 

 water. 



A genetic connection between the classes of volcanic 

 phenomena which have been mentioned is indicated by 

 many and various traces of simultaneity, and accompa- 

 nying transitions of the simpler and less powerful actions 

 into the more energetic and complicated ones. The 

 sequence in which I have arranged my materials, in the 

 mode of presentation which I have selected, is justified 

 by considerations of this nature. The magnetic activity 

 of our planet (of which the seat is not, indeed, to be 

 looked for in the molten interior, although, according to 

 Lenz and Bless, iron in a molten state is capable 

 of acting as the conductor of an electric or galvanic 

 current,) produces, when enhanced, a development of 

 light at the magnetic poles of the earth, or at least in 

 their vicinity. The first division of the present or tel- 

 luric volume of my work closed with this terrestrial lu- 

 minosity. I place at the opening of the second division, 

 or next in succession to this phenomena of a light-ex- 

 citing undulation of the ether caused by magnetic forces, 

 and first among volcanic phenomenon, that class of 

 volcanic activity which in its essential manifestations 

 acts only, as does the magnetic activity, dynamically : 

 exciting motions or undulations in the solid crust of the 

 earth, not producing or altering substances. Other not 

 essential, but in this point of view secondary, phenomena 

 (flames rising during earthquakes, eruptions of water, 

 and evolution of gas ( 228 ) following after the earthquake) 

 recall to the mind the phenomena of thermal waters 



