THE ORIGIN OF LUNAR VOLCANOES. 53 



examined, and the details of the powerful reactions dis- 

 played in this process still demand further study; but there 

 can be no doubt that the combination of silicic acid with 

 the base of the fluor spar is the fundamental reaction to 

 which the evolution of the volatile fluorides, etc., is mainly 

 due. 



A corresponding evolution of gases takes place in cosmi- 

 cal volcanic action, whenever silicic acid is fused in con- 

 tact with limestone or other carbonate, and a still closer 

 analogy is presented by the fusionof silicates in contact 

 with chlorides and oxides, in the absence of water. If the 

 composition of the Moon is similar to that of the Earth, 

 chlorides of sodium, etc., must form an important part of 

 its solid crust; they should correspond in quantity to the 

 great deposit of such salts that would be left behind if the 

 ocean of the Earth were evaporated to dryness. The only 

 assumptions demanded in applying these facts to the ex- 

 planation of the surface configuration of the Moon are, 1st, 

 that our satellite resembles its primary in chemical compo- 

 sition; 3d, that it has cooled down from a state of fusion; 

 and 3d, that the magnitude of the eruptions, due to such 

 fusion and cooling, must bear some relation to the quantity 

 of matter in action. 



The first and second are so commonly made and under- 

 stood, that I need not here repeat the well-known argu- 

 ments upon which they are supported, but may remark that 

 the facts above described afford new and weighty evidence 

 in their favor. 



If the correspondence between the form of a freely sus- 

 pended and rotating drop of liquid and that of a planet or 

 satellite is accepted as evidence of the exertion of the same 

 forces of cohesion, etc., on both, the correspondence be- 

 tween the configuration of the lunar surface, and that of 

 small quantities of fused and freely cooled earth-crust mat- 

 ter, should at least afford material support to the other- 

 wise indicated inference, that the materials of the Moon's 

 crust are similar to those of the Earth's, and that they have 

 been cooled from a state of fusion. 



I think I may safely generalize to the extent of saying, 

 that no considerable mass of fused earthy silicates can cool 



