TRUE VOLCANOES. 411 



of the pressure and of the power of conducting heat exercised 

 by various kinds of rock render it likely that the geothermal 

 degrees of depth increase in value in proportion as the depth 

 itself increases. 



Notwithstanding the very limited number of points at 

 which the fused interior of our planet now maintains an act- 

 ive communication with the atmosphere, it is still not uninr 1 

 portant to inquire in what manner and to what extent the 

 volcanic exhalations of gas operate on the chemical composi- 

 tion of the atmosphere, and through it on the organic life de- 

 veloped on the earth's surface. We must, in the first place, 

 bear in mind that it is not so much the summit-craters them- 

 selves as the small cones of ejection and the fumaroles, which 

 occupy large spaces and surround so many volcanoes, that 

 exhale gases ; and that even whole tracts of country in Ice- 

 land, in the Caucasus, in the high land of Armenia, on Java, 

 the Galapagos, the Sandwich Islands, and New Zealand ex- 

 hibit a constant state of activity through solfataras, naphtha 

 springs, and salses. Volcanic districts, which are now reckon- 

 ed among those which are extinct, are likewise to be regard- 

 ed as sources of gas, and the silent working of the subterra- 

 nean forces, whether destructive or formative, within them is, 

 with regard to quantity, probably more productive than the 

 great, noisy, and more rare eruptions of volcanoes, although 

 their lava fields continue to smoke either visibly or invisibly 

 for years at a time. If it be said that the effects of these 

 small chemical processes can be but little regarded, for that 

 the immense volume of the atmosphere, constantly kept in 

 motion by currents of air, could only be affected in its primi- 

 tive mixture to a very small extent through means of such 

 apparently unimportant additions,* it will be necessary to 



thickness of the earth's crust namely, 40,000 metres, or about 22 

 miles ; Elie de Beaumont (Systcmes de Montagues, t. iii., p. 1237) cal- 

 culates the thickness at about one fourth more. The oldest calcula- 

 tion is that of.Cordier, in mean value 56 geographical miles, an amount 

 which, according to Hopkins's mathematical theory of stability, would 

 have to be multiplied fourteen times, and would give between G88 and 

 860 geographical miles. I quite concur, on geological grounds, in the 

 doubts raised by Naumann in his admirable Ldirbuch der Geognosie 

 (vol. i., p. 62-64, 73-76, and 289), against this enormous distance of the 

 fluid interior from the craters of the active volcanoes. 



* A remarkable example of the way in which perceptible changes of 

 mixture are .produced in nature by very minute but continuous accu- 

 mulation is afforded by the presence of silver in sea-water, which was 

 discovered by Malaguti and confirmed by Field. Notwithstanding the 

 immense extent of the ocean and the trifling amount of surface pre- 



