STRUCTURE AND ACTION CF VOLCANOS. 359 



and albite), augite, hornblende, and sometimes interspersed 

 mica, and even quartz. Wherever the evidences of the first 

 eruption, the ancient structures — if I may use the expression 

 — remain complete, the isolated cone is surrounded, circus - 

 like, with a high wall of rock consisting of different super- 

 imposed strata, encompassing it like an outer sheath. Such 

 walls or circular inclosures are termed craters of elevation, 

 and constitute a great and important phenomenon, upon 

 which that eminent geologist, Leopold von Buch, from whose 

 writings I have borrowed many facts advanced in this trea- 

 tise, presented so remarkable a paper to our Academy five 

 years ago. 



Volcanos which communicate with the atmosphere by 

 means of fire-emitting mouths, such as conical basaltic hills, 

 and dome-like craterless trachytic mountains, (the latter being 

 sometimes low, like the Sarcouy, and sometimes high, like the 

 Chimborazo,) form various groups. Comparative geography 

 draws our attention, at one time, to small Archipelagos or 

 independent mountain-systems, with craters and lava streams, 

 like those in the Canary Isles and the Azores, and without 

 craters or true lava streams, as in the Euganean hills, and the 

 Siebengebirge near Bonn; at another time, it makes us ac- 

 quainted with volcanos arranged in single or double chains, 

 and extending for many hundred miles in length, cither 

 running parallel with the main direction of the range, as in 

 Guatimala, Peru, and Java, or intersecting its axis at right 

 angles, as in tropical Mexico. In this land of the Aztecs fire- 

 emitting trachytic mountains alone attain the high snow limit: 

 they are ranged in the direction of a parallel of latitude, 

 and have probably been upheaved from a chasm extending 

 over upwards of 420 miles, intersecting the whole continent 

 from the Pacific to the Atlantic. 



This crowding together of volcanos, cither in rounded 

 groups or double lines, affords the most convincing proof 

 that their action does not depend on slight causes located 



