EARTHQUAKES. 163 



f trie oarth may also be regarded as the subsidiary action 

 of a hon-telluric cause, by which an increased pressure 

 must be produced, either immediately against a solid, su- 

 perimpo^ed rocky arch ; or indirectly, when the solid mass 

 is separated, in subterranean basins, from the fused, fluid 

 mass by eiastic vapors. 



The nucleus of our planet is supposed to consist of un- 

 oxydized masses, the metalloids of the alkalies and earths. 

 Volcanic activity is excited in the nucleus by the access 

 of water and air. Volcanoes certainly pour forth a great 

 quantity of aqueous vapor into the atmosphere ; but the 

 assumption of the penetration of water into the volcanic 

 focus is attended with much difficulty, considering the op- 

 posing pressure* of the external column of water and of 



expresses himself, no more tide will be produced than if the ocean had 

 an indestructible covering of ice. The thickness of the solid, unfused 

 crust of the earth is calculated from the fusing points of the different 

 kinds of rock, and the law of the increase of heat from the surface 

 into the depths of the earth. I have already (Cosmos, vol. i., p. 45) 

 justified the assumption that at somewhat more than twenty geograph- 

 ical miles (21-J&J-, 25 English) below ihe surface a'heat capable of melt- 

 ing granite prevails. Nearly the same number (45,000 metres =24 

 geographical miles) was named by Elie de Beaumont (Geologie, edited 

 by Vogt, 1846, vol. i., p. 32) as the thickness of the solid crust of the 

 earth. Moreover, according to the ingenious experiments of Bischof 

 t>n the fusion of various minerals, of which the importance to the prog- 

 ress of geology is so great, the thickness of the unfused strata of the 

 earth is between 122,590 and 136,448 feet, or, on the average, 2l geo- 

 gi-aphical (24|- English) miles; see Bischof, Warmelehre des Innei-n 

 imsers Erdkorpers, p. 286 and 271. This renders it the more remark- 

 able to me to find that, with the assumption cf a definite limit between 

 the solid and fused parts, and not of a gradual transition, Hopkins, 

 from the fundamental principles of his speculative geology, establishes 

 the result that "the thickness of the solid shell can not be less than 

 about one fourth or one fifth (?) of the radius of its external surface" 

 (Meeting of British Association, 1847, p. 51). Cordier's earliest sup- 

 position was only 56 geographical (72 English) miles, without correc- 

 tion, which is dependent upon the increased 'press-are of the strata at 

 great depths, and the hypsometrical form of the surface. The thick- 

 ness of the solid part of the crust of the earth is probably very un- 

 equal. 



* Gay-Lussac, Reflexions sur les Volcans, in the .Annalcs de Clii- 

 mie et de Physique, tome xxii., 1823, p. 418 and 426. The author, 

 tt'ho, in company with Leopold von Buch and myself, observed the 

 great eruption of lava from Vesuvius in September, 1805, has the 

 merit of having submitted the chemical hypotheses t<x a strict criti- 

 cism. He seeks for the cause of volcanic phenomena in a "very en- 

 ergetic and still unsatisfied affinity between the substances, which a 

 fortuitous contact permits them to obey;" in general, he favors the 

 hypothesis of Davy and Ampere, which is now given up, "supposing 



