228 REACTION OF THE INTERIOR OF THE EARTH 



neys or vents from a long extended subterranean fis- 

 sure. The latter class of volcanoes, or those which form 

 lines, is again subdivided into those which rise as single 

 conical islands from the bottom of the sea, (in which case 

 they are usually parallel to, and at the foot of a chain of 

 primitive mountains), or they are elevated upon the highest 

 ridge of the primitive chain, of which they then form the 

 summits. ( 226 ) The Peak of Teneriffe, for example, is a 

 " central volcano ;" it is the center of the group to which 

 we refer the volcanic islands of Palma and Lancerote. The 

 grandest example of a continental volcanic "chain" is offered 

 by the great rampart of the Andes, extendingfrom the southern 

 part of Chili to the north-west coast of America, sometimes 

 forming a single range, sometimes divided into two or three 

 parallel branches, which are occasionally connected by nar- 

 row cross or transversal ridges. In this chain the proximity 

 of active volcanoes is always announced by the appearance of 

 certain kinds of rocks (dolerite, melaphyre, trachyte, an- 

 desite, and dioritic porphyry), breaking through and dividing 

 the primitive rocks, the transition slates and sandstones, and 

 the more recently formed strata. The constant recurrence 

 of this phenomenon led me long since to the belief that 

 these sporadic rocks were the seat of volcanic phenomena, 

 and determining conditions of volcanic eruptions. It was 

 at the foot of the majestic Tunguragua, near Penipe (on the 

 banks of the Rio Puela), that I first distinctly observed 

 mica schist (resting on granite) traversed by a volcanic rock. 

 In parts of the volcanic range of the new continent, where 

 the single volcanoes are nearest to each other, they shew a 

 certain mutual dependence and connection ; it even appears 

 that the volcanic activity has progressively advanced frr 

 centuries in certain directions, as in the province of 



