Notices respecting Ncoo Books. ' 297 



waters to the surface. And although, as Dr. Daubeny shows, we 

 find in springs remote from volcanos similar gaseous products to 

 those emitted by volcanos themselves, yet it is possible that the 

 production of these gases may be a general occurrence in the sub- 

 terranean regions, and in that case they would escape wherever 

 they can find an exit, either by the thermal spring or the volcanic 

 chimney. 



Concluding his summary of the facts of volcanic geology. Dr. 

 Daubeny proceeds in part 3 to draw conclusions from them as to 

 the causes of volcanos, the circumstances that influence the cha- 

 racter of their products, and the uses they fulfill in the oeconomy of 

 Nature. 



The two principal theories now maintained, as to the causes of 

 volcanic action, are the mechanical one and the chemical ; the for- 

 mer of which supposes that the earth's crust floats on a nucleus of 

 melted matter, the heat of which produces the expansive and explo- 

 sive forces of the volcano ; while the latter maintains that volcanic 

 heat is the direct effect of local chemical operations going on beneath 

 the district where the phsenomena are displayed. As we can never 

 hope to obtain any positive knowledge of the regions where volcanos 

 originate, and can only speculate as to their causes by studying a 

 multitude of very complicated eff"ects, it is probable (and the reflection 

 is consolatory) that these rival theories may continue to enliven the 

 meetings of the Geological Society in future ages, long after every 

 region of the world has been mapped, every section measured and 

 every fossil described. 



Dr. Daubeny has for many years been an advocate of the chemical 

 theory, and supposes volcanic heat to arise from the access of oxy- 

 gen to the metallic bases of the alkalies and earths. All who have 

 witnessed the combustion of potassium or of sodium by the contact 

 of water must admit that this is a possible hypothesis, and the ques- 

 tion therefore is merely whether that or the mechanical theory is 

 the more pi'obable. In proof of his opinion, Dr. Daubeny adduces 

 the vicinity of volcanos to the sea, the aqueous vapours which they 

 emit, and the hydrogen, nitrogen and otlier gases which indicate the 

 decomposition of air and of water, and the absorption of oxygen. He 

 lias argued this question with great ability on chemical grounds, and 

 has certainly made out a very good case, though we are not prepared 

 as yet to enlist ourselves under either banner. Both theories may 

 easily be true. Central heat and central fluidity may still exist as a 

 relic of the chaotic epoch, and would even be auxiliary to the che- 

 mical operations which may take place in the more superficial re- 

 gions of the earth's interior. 



Dr. Daubeny adopts to a great extent the views of Von Buch re- 

 garding " craters of elevation," that is, cavities formed by the up- 

 heaving and consequent separation of the superficial strata, as di- 

 Btinguislicd from " craters of eruption," caused by tlie accumulation 

 of ejected matter round tlie orifice of a volcano. Like all doctrines 

 which assume a controversial form, tlie elevation theory has l)cen 

 pushed too far by one party and unreasonably cried down by an- 



