Mr J. D. Dana on the Volcanoes of the Moon. 29 



der the same circumstances, if the elements arc present in 

 the material in fusion ; for the syenites alluded to are closely 

 allied rocks in texture and character. At a former meeting 

 of this Association, it was suggested by me that some regions 

 of granite peaks may have been centres of ancient igneous 

 action ; and their being surrounded or bordered by horn- 

 blendic rocks, seems to point to some actual analogy with 

 the trachytic centres and basaltic circumference of mountains 

 admitted to have been volcanic. 



The opinion that the nature of the resulting rock is di- 

 rectly connected with the nature of the rock which had enter- 

 ed into fusion, cannot be maintained if the above views be 

 true. On the contrary, it appears that while the result may 

 thus be varied, the mode of distributing minerals in a vol- 

 canic focus by the boiling process, may produce, from the same 

 material, rocks of a j)redominant feldspathic character in one 

 place, and rocks of a hornblendic or augitic character in other 

 places. Simple feldspathic granites may be fused and ejected 

 as feldspathic rocks, like those of porphyry dikes. But it is 

 an interesting fact, that the rock of most dikes is of the 

 augitic (or hornblendic) kind, like the dikes of volcanoes that 

 rise from sources in which the separating process could not 

 have been operating.* 



We also arrive at the important conclusion, that rocks per- 

 fectly compact in texture may be of subaerial origin, as we 

 have pressure from the fluid lavas themselves in the volcanic 

 focus. 



Another deduction proceeds from the facts stated ; — that 

 the same igneous rocks may occur of all ages, provided the 



* Mr Darwin has accounted for the distribution of feldspathic and augitic 

 rocks in volcanoes, on the ground of their different specific gravity. But with 

 this cause alone, the lower parts of the feldspathic peaks should be expected to 

 contain the heavier augitic material, which is not the case. He also argues 

 that the feldspar would rise in the fluid as crystals, and so the augite sink. 

 But we know, in the first place, that crystals do not appear till incipient solidi- 

 fication ; and, if the augite and feldspar were both distinct crystals, where 

 would be the fusion ? Aguin, the feldspar rocks are amorphous, except with a 

 very slow rate of cooling ; and how, then, can the existence of appreciable 

 crystals be assumed ? 



