GEOLOGY. 263 



elements. The low density of the sun, which is little greater than 

 that of water (0.252 that of the earth), would lead us to suppose the 

 existence there of a peculiar condition of things ; science has, how- 

 ever, as yet no means of appreciating the action of a heat so excess- 

 ive as that which is required to maintain the alkali-metals in a gas- 

 eous state, and it appears possible that if the temperature of the sun 

 were reduced to that of the earth, its density would also be approxi- 

 mated to that of our planet. However this may be, the analogies 

 of Leverrier's theory with the observations of geologists are too im- 

 portant, as showing the connection between the two great branches 

 of natural science, not to encourage geologists to further inquiry in 

 the same direction, and it is with this object in view that we have 

 been led to the following reflections. 



We admit a similar geological or chemical constitution for the va- 

 rious bodies of the solar system, and from this conclude that the phe- 

 nomena which have accompanied their formation and their successive 

 transformations must have been similar. Thus the planets and satel- 

 lites whose density is near to that of our earth may be supposed to 

 have passed through the different stages of liquid and solid incandes- 

 cence, of the successive liquefaction of portions of their gaseous en- 

 velopes, and to have finally been the seat of an organic creation. 



Of these planetary bodies the best known to us is the moon ; and we 

 shall now inquire to what extent our slight knowledge of it is in ac- 

 cordance with the observations made on our earth, and with the pres- 

 ent state of the sun as supposed by M. Leverrier. It is well known 

 that astronomers, so soon as they became possessed of good telescopes, 

 discovered mountains and plains, or seas, on the surface of the moon, 

 and the immediate application of these names shows the great re- 

 semblance which was supposed to exist between the surfaces of the 

 moon and the earth. It does not appear surprising that the form of 

 the lunar mountains should be met with among only a small number 

 of those on our planet, and physicists easily explain the greater ele- 

 vation and the steep declivities of the former by the comparatively 

 feeble action of the centripetal force at the moon's surface. But one 

 of the gravest objections to the idea of a common origin of the moon 

 and the earth is the apparent absence of water and air from the sur- 

 face of our satellite, thus seriously embarrassing those geologists who 

 attribute terrestrial volcanic phenomena to the intervention of these 

 expansible elements. 



If, however, we admit for the earth and the moon an identical and 

 simultaneous point of departure, we can understand that their cool- 

 ing has taken place at a rate nearly proportioned to their volume. 

 That of the moon bein^ about two hundredths the volume of the 

 earth, its temperature, if we admit an equal conductibility, will have 

 decreased with a rapidity fifty times greater, so that the geological 

 epochs of the moon will have been in the same proportion shorter 

 than the corresponding epochs on the earth, up to the time when the 

 solar heat began to be an appreciable element. The moon has then 

 advanced much more rapidly than the earth in the series of phenom- 

 ena through which both must pass, and we may therefore logically 

 suppose that our globe will one day offer the same general characters 

 as are now presented by the moon. 



