24 Mr J. D. Dana on the Volcanoes of the Moon. 



As there is little or no water in the moon to aid volcanic 

 action, sulphur has probably played an important part in its 

 igneous changes ; for this is not only a prevalent means of 

 igneous operations on our globe, but occurs in meteoric stones, 

 pyrites being one of their common constituents. We may, 

 therefore, believe that, wherever there is action in the moon, 

 sulphur and any other vapourizable material present, are con- 

 stantly escaping, either as simple vapour or in some gaseous 

 combination, and forming a very low covering over certain 

 portions. 



As we have observed, the existence of actual volcanic erup- 

 tion in the moon is still doubtful, and we must look to new 

 facts to settle the point. But we cannot doubt that the sur- 

 face in former periods has been everywhere in violent action, 

 and that its pools of fire were once measured by scores of 

 miles instead of by hundreds of yards, as with our existing 

 volcanoes. And many of these immense basins remain still 

 open for examination, presenting indications of the various 

 changes which accompanied the gradual decrease of igneous 

 action during the cooling in progress. A map of the moon, 

 if there is any truth in these views, should be in every geo- 

 logical lecture room ; for nowhere can we have a more com- 

 plete or more magnificent illustration of volcanic operations. 

 Our own sublimest volcanoes would rank among the smaller 

 lunar eminences ; and our Etnas as but spitting furnaces. 



In continuation, I would ask attention to some thoughts 

 bearing on our own planet, which are suggested by this study 

 of the moon's surface. 



If the earth was once a melted globe, it must have passed 

 through the same phases as the moon, with this very import- 

 ant difference, that the whole surface during its progress was 

 subject to the denuding action of waters, and from the first 

 had valleys and sedimentary rocks in progress. It must have 

 had originally its boiling pools of vast extent ; which, as tlie 

 action decreased in violence would more or less gradually 

 contract. Are there any remains of these great craters \ Or 

 have they disappeared by a decrease in the volcanic action 

 and thus graduated into existing mountains, or have they 



