A COMTARISON OF THE FEATURES OF THE EARTH AND THE MOON. II 



accept the commonly adopted view as to the nature of the prevaiHng topo- 

 graphical features of that sphere and regard them as essentially volcanic, i. e., as 

 due mainly to the expulsion of heated vapors or gases from the interior of the 

 sphere, we have a basis on which to found a determination of that age sufficiently 

 accurate to serve our immediate purpose. 



It appears eminently probable that the lunar surface must have attained to 

 something like its present condition long before the earth came to the state in 

 which its igneously fluid mass was crusted over. And this for the following 

 reasons : At the time when the material of the moon and earth separated 

 from the previously united mass we have to believe that the amount of heat 

 they severally contained was in general proportionate to the mass of each 

 body. Now the mass of the moon is to that of the earth as one to eighty, and 

 its diameter about as one to four. P'rom this, by the well-known law of cooling 

 bodies, it follows that the moon must have acquired a permanent rigid crust, if 

 indeed it did not become entirely frozen, long before the earth ceased to have a 

 molten surface. There are too many doubtful elements in the computation to 

 make any seemingly accurate reckoning trustworthy, but it appears altogether 

 likely that the moon cooled far beyond the point where volcanic action was pos- 

 sible ages before the earth's surface could have frozen or perhaps have passed 

 from the gaseous to the fluid state. 



At present all the volcanic action of the earth is apparently limited to the 

 sea-floor or regions within three hundred miles of the shore ; effectively to regions 

 where the central heat is brought upwards into strata containing water laid in 

 them when they were deposited ; the rise of the heat being due to the slow 

 conductivity of the imposed bed.s. There is reason to believe that since the 

 earliest recorded ages the earth has mainly, if not altogether, depended on such 

 action for the volcanic outbreaks which have occurred upon it. While there may 

 in this particular matter be some reason for doubt, there is none as to the fact 

 that if the so-called lunar volcanoes are due to the central heat of that sphere, 

 they must have been shaped before the crust of the earth was formed, or long 

 before the earliest geological records. It has, however, been suggested by G. K. 

 Gilbert ' and others that what appear to be volcanoes on the moon are not really 

 such, but are, in effect, punctures caused by the falling of large meteorites or 

 bolides. This interesting suggestion commends itself at first sight as a possible 

 explanation of the pits on the moon, structures which differ in many regards from 

 those due to terrestrial volcanic action, in that they are often of much greater diam- 

 eter, have relatively much smaller encircling cones, and show little, if any, clear 

 evidence of lava flows, or ash showers, proceeding from them. As I propose fur- 

 ther on in this paper to discuss the question of their nature in more detail, I shall 

 now give only in brief the reasons why, as it seems to me, the hypothesis that 

 they were caused by bodies falling from the sky is not verified. 



It is to be noted that these so-called volcanoes of the moon, vulcanoids, as I 

 shall term them, have generally very steep walls around their crater-like pits ; 

 ' See Bull. Phil. Soc. of IVasliingloii, vol. 12, p. 241, ei seq. 



