236 THE WORLD. 



of simple mechanical action, produced in a manner we Cannes 

 here describe. 



Volcanoes are found distributed all over the surface of the earth, 

 though more prevalent in some portions than in others. Many 

 of the islands in the Pacific and Atlantic are of volcanic origin; 

 perhaps the majority of them. In some parts of the earth vol- 

 canoes stand alone, but they are mostly connected with extensive 

 mountain ranges, extending in a linear direction, and we may 

 select three distinct regions of subterranean disturbance. The 

 most extensive is that of the Andes.. Along the whole western 

 shores of North and South America, extends a lofty mountain 

 chain, remarkable not only for its position, but also for its 

 collosal form, the nature of the masses of which it is com- 

 posed, and of the materials ejected. Along the whole extent o-f 

 this chain, volcanoes occur, and between the 4Gth deg. of south 

 latitude to the 27th deg. is a line of volcanoes so uninterrupted 

 that scarcely a degree is passed without the occurrence of one of 

 these in an active state ; about twenty now active are enumera- 

 ted in this space, and doubtless there are very many more which 

 have been active at a recent period. When we remember how 

 long a time Vesuvius had remained quiet, before it again renewed 

 its activity, and overwhelmed the cities of Herculaneum and 

 Pompeii, we can readily admit that the number of volcanic vents 

 or craters is much greater than really is now apparent. The im- 

 mense height of the volcanic mountains of the Andes and Cor- 

 dilleras is very remarkable, and the craters are all formed by 

 bursting through porphyritic rock, or igneous unstratified rock, 

 containing crystals of feldspar. Some of the loftiest summits are 

 composed of trachyte, a rock of igneous origin, unstratified and 

 allied to the trap rocks, such as basalt, greenstone, &c. On the 

 summits are found large quantities of obsidian, or dark green vol- 

 canic glass, pumice stone,, and tuff' formed out of cinders, and 

 fragments of lava cemented together. 



It appears highly probable that a chain of volcanic vents ex- 

 tends quite around the globe, in the general direction north and 

 south. A lofty chain of mountains was discovered by Capl. J. 

 C. Ross, in the Antarctic regions in year 1841, at a distance of 



