Mr J. D. Dana on the f^olcanoes of the Moon. 25 



been swept away by the changes of time \ M. Von Buch has 

 described a circular area on the island of Palma, one of the 

 Canaries,* six miles in diameter, which has been compared 

 to a lunar crater, with some appearance of reason. It is, in 

 fact, hardly twice the diameter of Kilauea, which it other- 

 wise resembles. On Mauritius there is a similar area, fifteen 

 miles in diameter, surrounded by precipitous walls composed 

 of the edges of strata dipping outward-t Either it is a vol- 

 canic mountain whose centre has fallen in, as suggested by 

 M. Eailly, or it is the remains of a great pit-crater. I merely 

 state the fact without expressing an opinion. Other instances 

 might be mentioned, but this will suffice. At the present 

 period, few active boiling pits remain, and Kilauea is the 

 only one whose characters have been well determined. The 

 surface fires of the globe have so far subsided in action, 

 that in nearly every existing volcano, cinder ejections charac- 

 terize the action at summit, and eruptions of lavas in streams 

 are confined to fissures through the sides and flanks of the 

 mountain. 



II. We are led by the facts displayed, to remark also on 

 the origin of the mineral constitutio7i of igneous rocks. 



It has been a difficult problem for solution, why volcanic 

 regions should have a centre of solid feldspathic rocks, un- 

 sfcratified and compact, while the exterior consisted mainly of 

 basaltic lavas. Scrope, Von Buch, and other writers on vol- 

 canoes, have mentioned instances of this structure ; and it 

 seems to characterize generally the large volcanic mountains. 

 It is well exhibited when the elevations are cut through by 

 gorges ; and when not, the clinkstone appears often at the 

 summit of the cone or dome. The explanations we here ven- 

 ture, proceed on two principles : 



1. The motion which belongs to a boiling fluid. 



2. The less fusibility of feldspar than the other ingredients. 

 In the great boiling pools, there will necessarily be a rising 



of the fluid, in the hotter part, and a flow away towai^ls either 

 side, producing a kind of circulation. This is no h) pothesis, 



* Desc. rhys. des lies Canaries. Paris, 1836, p. 281. 

 t Darwin's Volcanic Islands. London, 1844, p. 30. 



