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



We hence comprehend the rapid cooling which character- 

 izes ejected lavas, /«?r onltf a part of the material is in complete 

 fusion. 



The actual nature of the cooled igneous rock may be'more 

 correctly understood, if we consider that the minerals pre- 

 sent will depend, not only on heat and pressure, and the 

 causes above alluded to, but also on rate of cooling. The 

 effect of slow cooling is exemplified in the feldspathic centre 

 of a volcanic mountain. Being wholly enclosed by rocks, the 

 heat of fusion passes oif slowly, and owing to the pressure of 

 its own superincumbent portions, the rock is compact. What- 

 ever augite may be present, instead of appearing as augite, 

 will take the form of hornblende, a mineral which requires 

 slow cooling, and differs from augite in the crystalline form 

 which it thus receives. In corroboration of this statement, 

 hornblende is common in trachytes and such feldspathic 

 rocks. The same remarks apply to mica : and other mine- 

 rals also may form according to the elements present. Chry- 

 solite is not met with : it occurs only where there is a more 

 rapid rate of cooling, as in the formation of ordinary basaltic 

 rocks or lavas. 



Farther, we observe, that with a still more gradual rate of 

 cooling, the whole feldspathic rock becomes crystalline in 

 texture like a granite or syenite, and it is well known that 

 granite-like or syenitic rocks or peaks occur in some volcanic 

 regions, whose interior has been laid open by denudation. 

 Many minerals, too, might crystallize under these circum- 

 stances, which, with more rapid cooling, would not be dis- 

 tinguishable. 



The boiling process in a large volcano, therefore, in con- 

 nection with the circumstances of temperature, rate of cool- 

 ing, the fusibilities of different minerals, and the other causes 

 alluded to, will account for the various features, positions, 

 and relations of igneous rocks, and for many facts relating 

 to the distribution of igneous minerals.* We may hence rea- 

 sonably infer that granite and granite minerals may form un- 



* The general causes referred to, act under the guiding laws of crystallo- 

 geny, which laws regulate the particular positions of minerals according to the 

 principles exemplified in segregations or radiated crystallizations, and tlxe lanii- 

 jiated or ck'tivablc t^lruclurc of igneous rocke;. 



