OF VOLCANIC ROCKS. T9 



canoes of the Indian Archipelago. Active volcanoes have also been found to be par- 

 ticularly numerous in those regions where the narrow terminations of two continents 

 verge towards connection, as is the case in Central America, between Alaska and 

 Kamtschatka, and between Australia and Farther India. The mode of distribution of 

 extinct volcanoes has been little investigated. But as they are far more numerous 

 than active vents, and as volcanic action has completely ceased in extensive regions, a 

 perfect understanding of the distribution of volcanic activity on the globe can only be 

 acquired when they shall be fully taken into consideration. The general laws of their 

 distribution appear not to differ from those relating to active volcanoes. They, too, 

 have been evidently dependent upon the course of the sea-coasts which existed at the 

 time of their activity, and were in many places quite different from those of the pres- 

 ent epoch. But we must note, besides, this additional feature of extinct volcanoes, 

 that among the theaters of their grandest and most extensive displays are certain 

 countries which were formerly covered with large inland basins filled with salt water, 

 and form now mostly elevated table-lands on which may still be seen the remnants of 

 those former inland seas. Volcanoes so situated form often crowded groups occurring 

 at distances of more than five hundred miles from the sea-coasts which existed con- 

 temporaneously with their activity. To them belong the numerous extinct volcanoes 

 on the plateau between the Sierra Nevada and the Rocky Mountains, a region which 

 has probably had no equal in regard to the extent in which volcanic activity has taken 

 place, as it comprises the States and Territories of Oregon, Idaho, Montana, Utah, Col- 

 orado, Nevada, California. Arizona, New Mexico, and continues southerly to the high 

 lands of Mexico. Other volcanoes of high table-lands are those of the Mongolian 

 Desert, the Thian-Shan, Armenia and, probably, eastern Africa. The volcanoes of all 

 these regions are extinct, with the exception of a few in the southern part of the pla- 

 teau of Mexico, which are in action, and some others which still exhibit solfataric phe- 

 nomena. 



Interesting as would be a map embracing all active and extinct volcanoes, it 

 would give only the outlines of the distribution of those larger accumulations of volcanic 

 rocks which were produced by massive eruptions, and have ordinarily attracted little 

 attention. 27 Geological maps being often imperfect as regards them, it is not yet pos- 

 sible to distinguish any other features of their distribution than those mentioned in 

 reference to volcanoes, as they can always be found in the vicinity of these. We may 

 however add, that massive accumulations may eventually be found near the summits 

 of mountain ranges, where volcanoes do ordinarily not occur. 



27 This may be exemplified in regard to the Andes of North and South America. Humboldt (Cosmos, vol. iv) 

 distinguishes five groups of volcanoes, separated by non-volcanic regions. They are mentioned by him as follows : 1st, vol- 

 canic group of Mexico, nearly five hundred miles in length, but occupying scarcely more than one degree of latitude, on 

 account of its direction from east to west; 2d, three hundred and fifty miles free from volcanoes; 3d, group of Central 

 America, having a length of over eight hundred miles ; 4th, seven hundred and thirty-five miles non-volcanic ; 5th, group of 

 New Granada and Quito, five hundred and fifty-five miles in extent ; 6th, the longest interval without volcanoes, being 1,133 

 miles ; 7th, group of volcanoes of Peru and Bolivia, extending over five hundred miles ; 8th, a non-volcanic space of six 

 hundred and twenty miles :9th, the group of Chile, the most extensive range of volcanoes of America, being 1,143 miles in 

 length. Summing up, Humboldt takes about 3,000 miles to be the aggregate extent in length of the volcanic regions, 2,838 

 miles that of the intermediate spaces which are free from volcanoes. If all the volcanic rocks occurring in the Andes 



,(1 17) 



