PLANETESIMAL THEORIES OP THE EARTHS ORIGIN. 205 



an explanation of this difference. I think it is very difficult to 

 believe they can be volcanoes at all, and I am glad to think there 

 are theories to account for the mass of rings on the moon's 

 surface. 



The Secretary. — Sir Robert Ball and myself and my son paid 

 a visit several years ago to the Auvergne district of Central France, 

 a district of recently extinct volcanoes, and he made that journey 

 with the special purpose of observing the extinct volcanoes and their 

 apparent similitude to those of the moon. I am sure Sir Robert 

 Ball is a strong believer in the crater-like forms on the moon's 

 surface as being volcanic. They are very deep depressions because 

 the shadows are deep. The terrestrial ones are smaller than the 

 moon's, but some of those in the Pacific Ocean, the great volcanic 

 islands — are of enormous size — six or seven miles in diameter. 



Mr. Rouse. — It occurred to me that the impression made by a 

 snowball upon another ball, or upon a wall for that matter, would 

 not have been like that of the volcanic walls on the moon, because 

 there would have been an inward slope as well as an outward, whilst 

 they present the appearance of a perpendicular wall without. If 

 any soft body is hurled against another there will be an inner slope 

 of considerable deposit. There will be an inner very considerable 

 slope greater than the outer. 



Then it has also occurred to me that if the moon itself was in at 

 all a soft condition, as we may suppose it was at that time, that 

 there would be also a depression in the moon — not only a flat 

 appearance which looks like the continued level of the moon inside 

 the volcanic wall, but there would be a hollow. 



The Secretary. — There is one difference between the extinct 

 volcanoes of Auvergne and those of the moon. In Auvergne the 

 lava flows break down the walls of the circle, which is generally 

 formed of volcanic ash, but through which molten lava is coming up 

 and filling the great bowl gradually up. It has broken down that 

 rim in some places of least resistance, and then you have a stream 

 flowing out for several miles, and so little covered with vegetation 

 that you might think it was only a few years since they had 

 ceased to flow. 



Rev. John Tuckwell, M.R.A.S.— It will not be possible for 

 us to spend time enough to discuss this nebular hypothesis to any 

 thing like its full extent. 



