Mr. R. P. Greg on the Lunar Origin of Aerolites. 433 



Wc cannot reasonably suppose that lunar volcanoes have 

 ejected enormous masses of iron, whether in a pure or oxidized 

 state, when iron occurs in such small quantity? and so rarely as 

 the product of terrestrial volcanoes, and then most frequently de- 

 posited bv sublimation. The density of the moon, as given by 

 Dr. Smith, is only 3'6, while that of the earth is o'6 ; this ren- 

 ders it still more improbable that substances of greater density, 

 as the metals, are more abundantly ejected from the volcanoes in 

 the moon, tiian from those of the earth, the latter body having 

 the greater average density, I would also observe, that the 

 metal nickel, present in almost all known aerolites and iron 

 masses, has ne\'er yet been observed as a direct product of our 

 volcanoes ; this is not, however, an argument of much moment 

 one way or the other, especially as that metal, in the form of 

 red nickel, is known to occur in grauwacke at Reichelsdorf in 

 Hessia*. 



The argument, however, against the first proposition of ]1r. 

 Smith, that " all meteoric masses have a community of origin," 

 (and militating therefore against his conclusion that that com- 

 mon origin is the moon's volcanoes,) which may be most forcibly 

 illusti'ated, is where Me consider the case of an iron mass weigh- 

 ing from 10 to 20 tons, as those from Durango and Rio de la 

 Plata ; for it is at once evident that no ordinary initial volcanic 

 force could ever project such ponderous masses beyond the point 

 or limit of the mutual attractions of the moon and earth. The 

 calculations which have been made respecting the velocity of 

 stones jjrojected from ^Etna or Teneriffe, are based on the sup- 

 jjosition that such stoneS are of moderate size, having a density of 

 only 3'0, or nearly three times less than that of iron. 



I believe I am speaking within bounds when I state, that no 

 stone weighing more than 100 lbs. has ever been ejected from 

 the above-named volcanoes by a force, which, if exerted at the 

 moon's surface, would allow of its reaching the desired point of 

 neutralized attractions. 



Dr. Daubeny states that the stones which overwhelmed Pompeii 

 did not weigh more than 8 lbs. ; and I myself can bear witness 

 that the largest erupted blocks which crop out from under the 

 lava of Mount Somma, and much nearer the central cone there- 

 fore than Pompeii, seldom exceeded .50 lbs. in weight. It can 

 assuredly then only be stones of very moderate size, say of some 

 5 or 10 lbs., which could in any case, reasonaljly and practically 

 speaking, reach or pass that limit where the superior attraction 

 of the moon herself is lost. 



It surely would make a material difFerence in our calcula- 



• I have a fine s))ecimcn in my cabinet of minerals from that locality, 

 the matrix very much resembUng some meteoric stones. 



