220 STRUCTURE AND MODE OF ACTION 



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

 sometimes at its extremity : such is Pichincha, situated 

 between the Pacific and the city of Quito, and which 

 acquired celebrity in connection with Bouguer's earliest 

 barometric formulae, and such are the volcanos which rise 

 in the elevated Steppe de los Pastos, itself ten thousand 

 (10657 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 struc- 

 ture, or scaffolding remain 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 elevation," a 

 great and important phenomenon, concerning which a me- 

 morable treatise was presented to our Academy five years 

 ago (i. 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. 



Yolcanos which communicate with the atmosphere through 

 permanent 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 without craters and without 



