EARTHQUAKES. 1 69 



of the earth, may also be regarded as the subsidiary action 

 of a non -telluric cause, by which an increased pressure must 

 be produced, either immediately against a solid, superim- 

 posed rocky arch ; or indirectly, when the solid mass is 

 separator!, in subterranean basins, from the fused, fluid 

 mass by elastic vapours. 



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

 oxidised 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 vapour into the atmosphere ; but the 

 assumption of the penetration of water into the volcanic 

 focus is attended with much difficulty, considering the 

 opposing pressure 11 of the external column of water and 



miles (21 T 6 Q, 25 English) below the surface, a heat capable of melting 

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

 graphical miles) was named by Elie de Beaumont (Geologic, 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 Bischot 

 on the fusion of various minerals, of which the importance to the pro- 

 gress 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 21-i geo- 

 graphical (24 g English) miles; see Bischof, Warmelehre des Innern unsers 

 Erdkorpers, pp. 286 and 271. This renders it the more remarkable to 

 me to find that, with the assumption of 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 cannot 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 supposition was 

 only 56 geographical (72 English) miles, without correction, which is 

 dependent upon the increased pressure of the strata at great depths, 

 and the hypsqmetrical form of the surface. The thickness of the solid 

 part of the crust of the earth is probably very unequal. 



11 Gay Lussac, Reflexions sur les Volcans, in the Annales de Chimie et 

 de Physique, tome xxii, 1823, pp. 418 and 426. The author, who, in 

 company with Leopold von Buch and myself, observed the great erup- 

 tion of lava from Vesuvius in September, 1805, has the merit of having 

 submitted the chemical hypotheses to a strict criticism. He seeks for 

 the cause of volcanic phenomena in a "very energetic and still unsatis- 

 fied affinity between the substances, which a fortuitous contact permits 

 them to obey ;" in general he favours the hypothesis of Davy and 

 Ampere, which is now given up, " supposing that the radicals of silica, 

 alumina, lime, and iron are combined with chlorine in the interior of 

 the earth ," and the penetration of sea water does not appear to him to 

 be improbable unde^ certain conditions (pp. 419, 420, 423, and 42t>), 



