Mr J. D. Dana on the Volcanoes of (he Moon, 23 



through with explosive violence, producing an alternation, it 

 may be, of cinder and lava eruptions, or cinder eruptions 

 alone at summit, and raising up conical pointed peaksi These 

 different phases, in connection with fissure eruptions and up- 

 liftings from contraction, from both which causes ridges 

 might result, give us a complete and comprehensive view of 

 the origin of the moon's features; 



Are the lunar craters still active ? To a very great extetlt 

 the surface has evidently cooled ; and whether there are any 

 active points is a matter of doubt. But without admitting 

 igneous action at the present time in some parts, how can 

 the facts mentioned with regard to the difference in the light 

 of different portions of the moon's surface (p. 17) be satis- 

 factorily explained \ This difference may possibly be partly 

 accounted for on the ground (borne out by Kilauea), that the 

 bottom of a crater may have a smoother surface than the de- 

 clivities or plains exterior. Perhaps also there is something 

 attributable to a difference of material, though this is not 

 probable. If these explanations are received as sufficient for 

 the craters, they fail of satisfying Us with regard to the light 

 streaks which are so remarkable about some cones — coursing 

 over ridges and depressions without interruption. The fact 

 of illuminated walls to a crater when the bottom is not illu- 

 minated, and the general diffusion of light when one or more 

 bright areas of small extent may be distinguished, are also 

 points not easily understood on the above suppositions. May 

 it not be, that we should attribute some of the instances of 

 lighted areas to a covering of vapours from the igneous ac- 

 tion beneath \ The light streaks are not depressions, and, 

 therefore, hot broad fissures having the great width they ex- 

 hibit ; but they may be regions containing many fissures from 

 which vapours are escaping, and by the coalescence of such 

 areas, the summit of a crater like Eider might appear illu- 

 minated. Such vapours might so cover the bottom of a cra- 

 ter that the walls would appear brightest. Moreover, they 

 might leave the cones within a crater still distinct ; for if 

 spread out at a height of 500 feet above the bottom, they would 

 still, in many instances, be more than 10,000 feet below the 

 summit, and far below, too, the tops of interior peaks. 



