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



Whence all this close resemblance to the lunar craters, 

 while other volcanoes are so different \ It arises from the 

 fact, that the action at Kilauea is simply hailing^ owing to 

 the extreme fluidit}' of the lavas. The gases or vapours which 

 produce this appearance of active ebullition, escape freely in 

 small bubbles with little commotion, like the jets over boil- 

 ing water ; while at Vesuvius, and other like cones, they 

 collect in immense bubbles before they accumulate force 

 enough to make their way through ; and, consequently, the 

 lavas in the latter case are ejected with so much violence, 

 that they rise to a height often of many thousand feet and 

 fall around in cinders. This action builds up the pointed 

 mountain, while the simple boiling of Kilauea makes no cin- 

 ders and no cinder cones. Still, altliough the lavas of this 

 crater are Aot thrown to a great height, they may make cones 

 of any angle, even by overflowing alone ; especially by small 

 or partial overflowings, which melt together, cooling at the 

 same time rapidly. They thus sometimes raise a steep rim 

 around a pool. This point has been well presented by M. 

 C. Prevost,* and in another place we shall mention many 

 facts in illustration of it. 



If the fluidity of lavas, then, is suflicient for this active 

 ebullition, we may have boiling going on over an area of an 

 indefinite extent ; for the size of a boiling lake can have no 

 limits except such as may arise from a deficiency of heat. 

 The size of the lunar craters is, therefore, no mystery. Nei- 

 ther is their circular form of difficult explanation ; for a boil- 

 ing pool necessarily, by its own action, extends itself circu- 

 larly around its centre.t The combination of many circles, 

 and the large sea-like areas, are as readily understood. % 



* Bulletin de la Soc. Geol. de France, xi., 1839 a 1840, p. 183. 



T Mr Na3myth suggests that the quietness of tho lunar atmosphere may ac- 

 count for the regularity of the circles. 



+ M. Rozet observes, in his article referred to, that the moon's craters do not 

 resemble those of our volcanoes : and he explains them by supposing, that 

 during the cooling of the moon's surface there were ivhirlpools or circular flow- 

 ings, which carried the scoria from the centre to the circumference, and thus 

 accumulated the enclosing ridges. We see no cause for the existence of whirl- 

 pools ; nor for such a result from vortical movements. 



