380 STRUCTURE, AND MODE OF ACTION 



earth, which are situated on long chains of mountains with serrated 

 crests, and not even always on the middle of the ridge, but some- 

 times at its extremity: such is Pichincha, situated between the 

 Pacific and the city of Quito, and which acquired celebrity in con- 

 nection with Bouguer's earliest barometric formulae, and such are 

 the volcanos which rise in the elevated Steppe de los Pastes, itself 

 ten thousand (10,657 English) feet high. All these summits, 

 which are of various shapes, consist of trachyte, formerly called 

 Trap-porphyry: a granular, vesicular rock composed of different 

 kinds of feldspar (Labradorite, Oligoklase, and Albite), augite, 

 hornblende, and sometimes interspersed mica, and even quartz. In 

 cases where the evidence of the first outburst or eruption, or I might 

 say where the ancient structure or scaffolding remains entire, the 

 isolated conical mount is surrounded by an amphitheatre or lofty 

 circular rampart of rocky strata superimposed upon each other. 

 Such walls or ring-formed ramparts are called " craters of eleva- 

 tion," a great and important phenomenon, concerning which a 

 memorable treatise was presented to our Academy five years ago 

 (4. e. in 1818), by the first geologist of our time, Leopold von 

 Buch, from whose writings I have borrowed several of the views 

 contained in the present discussion. 



Volcanos which communicate with the atmosphere through per- 

 manent openings, conical basaltic hills, and craterless trachytic 

 domes, sometimes as low as Sarcouy, sometimes as lofty as the 

 Chimborazo, form various -groups. Comparative geography shows 

 us sometimes small clusters or distinct systems of mountains, with 

 craters and lava : currents in the Canaries and the Azores, and with- 

 out craters and without lava-currents, properly so called, in the 

 Euganean hills and the Siebengebirge near Bonn; and at other 

 times the same study describes to us . volcanos arranged in single 

 or double lines extending through many hundred leagues in length, 

 these lines being either parallel to the direction of a great chain of 

 mountains, as in Guatimala, in Peru, and in Java, or cutting it 

 transversely or at right angles, as in tropical Mexico. In this land 

 of the Aztecs the fire-emitting trachytic mountains are the only 

 ones which attain the elevation of the lofty region of perpetual 

 snow; they, are ranged in the direction of a parallel of latitude, and 



