TRUE VOLCANOES. 311 



noes in Middle and Northern Italy, and in the south of 

 France. 



This remarkable contrast between the narrow and un- 

 doubted lava streams of Auvergne thus early recognized, 

 and the often too absolutely asserted absence of any effusion 

 of lava in the Cordilleras, occupied me seriously during the 

 whole period of my expedition. All my journals are full of 

 considerations upon this problem, the solution of which I 

 long sought in the absolute elevation of the summits and in 

 the vastness of the circumvallation, that is to say, the sink- 

 ing of trachytic conical mountains from mountain plains of 

 eight or nine thousand (8500 9600 English) feet in eleva- 

 tion, and of great breadth. We now know, however, that a 

 volcano of Quito, 17,000 feet in height, which throws out 

 scoriae (that of Macas), is uninterruptedly much more active 

 than the low volcanoes Izalco and Stromboli ; we know that 

 the eastern dome-shaped and conical mountains, Antisana 

 and Sangay, have free slopes toward the plains of the Napo 

 and Pastaza ; and the western ones, Pichincha, Iliniza, and 

 Chimborazo, toward the affluents of the Pacific Ocean. In 

 many, also, the upper part projects without circumvallation 

 eight or nine thousand feet above the elevated plateaux. 

 Moreover, all these elevations above the sea-level, which is 

 regarded, although not quite correctly, as the mean elevation 

 of the earth's surface, are certainly inconsiderable as com- 

 pared with the depth which we may ass"ume to be that of 

 the seat of volcanic activity, and of the necessary tempera- 

 ture for the fusion of rock-masses. 



The only phenomena resembling narrow lava eruptions 

 which I discovered in the Cordilleras of Quito are those 

 presented by the colossal mountain Antisana, the height of 

 which I determined to be 19,137 feet (5833 metres) by a 

 trigonometrical measurement. As the structure furnishes 

 the most important criterion here, I will avoid the system- 

 atic denomination lava, which confines the idea of the mode 

 of production within too narrow limits, and make use, but 

 quite provisionally, of the names "rock-debris" (Fektrummern) 

 or " detritus dikes " (Sclmttw alien, trainees de masses volcan- 

 iques). The mighty mountain of Antisana, at an elevation 

 of 13,458 feet, forms a nearly oval plain, more than 12,500 

 toises (79,950 feet) in long diameter, from which the portion 

 of the mountain covered with perpetual snow rises like an 

 island. The highest summit is rounded off and dome-shaped. 

 The dome is united by a short, jagged ridge, with a truncat- 



