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considerable ought to be their craters. In fact, 

 there are immense volcanoes in the Andes, which 

 have but very small openings ; and we might 

 establish it as a geological principle, that the 

 most colossal mountains have craters of little 

 extent at the summits, if the Cordilleras did not 

 offer many instances * to the contrary. I shall 

 have occasion, in the progress of this work, to 

 cite a number of facts, which will throw some 

 light on what may be called the external struc- 

 ture of volcanoes. This structure is as varied 

 as the volcanic phenomena themselves : and in 

 order to raise ourselves to geological conceptions 

 worthy of the greatness of nature, we must set 

 aside the idea, that all volcanoes are formed 

 after the model of Vesuvius, Stromboli, and 

 Etna. 



The external edges of the Caldera are almost 

 perpendicular. Their appearance is somewhat 

 like the Somma, seen from the Atrio del Cavallo. 

 We descended to the bottom of the crater on a 

 train of broken lava, from the eastern breach of 

 the enclosure. The heat was perceptible only 

 in a few crevices, which gave vent to aqueous 

 vapours with a peculiar buzzing noise. Some 

 of these funnels or crevices are on the outside of 

 the enclosure, on the external brink of the para- 



* The great volcanoes of Cotopaxi and Rucupichincha 

 have craters, the diameter of which, according to my measure- 

 ments, exceed 400 and 700 toises. 



