VOLCANOES. 239 



active volcanoes is always manifested in the chain of the An- 

 des by the appearance of certain rocks (as dolerite, melaphyre, 

 trachyte, andesite, and dioritic porphyry), which divide the so- 

 called primitive rocks, the transition slates and sandstones, and 

 the stratified formations. The constant recurrence of this 

 phenomenon convinced me long since that these sporadic rocks 

 were the seat of volcanic phenomena, and were connected with 

 volcanic eruptions. At the foot of the grand Tunguragua, 

 near Penipe, on the banks of the Rio Puela, I first distinctly 

 observed mica slate resting on granite, broken through by a 

 volcanic rock. 



In the volcanic chain of the New Continent, the separate 

 volcanoes are occasionally, when near together, in mutual de- 

 pendence upon one another ; and it is even seen that the vol- 

 canic activity for centuries together has moved on in one and 

 the same direction, as, for instance, from north to south in the 

 province of Quito.* The focus of the volcanic action lies be- 

 low the whole of the highlands of this province ; the only 

 channels of communication with the atmosphere are, howev 

 er, those mountains which we designate by special names, as 

 the mountains of Pichincha, Cotopaxi, and Tunguragua, and 

 which, from their grouping, elevation, and form, constitute the 

 grandest and most picturesque spectacle to be found in any 

 volcanic district of an equally limited extent. Experience 

 shows us, in many instances, that the extremities of such 

 groups of volcanic chains are connected together by subterra- 

 nean communications ; and this fact reminds us of the ancient 

 and true expression made use of by Seneca,! that the igneous 

 mountain is only the issue of the more deeply-seated volcanic 

 forces. In the Mexican highlands a mutual dependence is 



* Humboldt, Geognost. Beobach, fiber die Vulkane des Hoclilandes von 

 Quito, ill Poggend., Annul, der Physik, bd. xliv., s. 194. 



t Seneca, while he speaks veiy clearly regarding the problematical 

 sinking of iEtna, says in his 79th letter, " Though this might happen, 

 not because the mountain's height is lowered, but because the fires are 

 weakened, and do not blaze out with their former vehemence ; and for 

 which reason it is that such vast clouds of smoke ai'e not seen in the 

 day-time. Yet neither of these seem incredible, for the mountain may 

 possibly be consumed by being daily devoured, and the fire not be so 

 large as formerly, since it is not self-generated here, but is kindled in 

 the distant bowels of the earth, and there rages, being fed with con- 

 tinual fuel, not with that of the mountain, through whicla it only makes 

 its passage." The subterrauean communication, "by galleries," be- 

 tween the volcanoes of Sicily, Lipari, Pithecusa (Tschia), and Vesuvius, 

 "of the last of which we may conjecture that it formerly burned and 

 presented a fiery circle," seems fully understood by Strabo (lib. i., p. 

 247 and 248). He terms the whole district " sub-igneous." 



