THE EEVOLUTIONS OF THE CRUST OF THE EARTH. 297 



would hardly amouut to the five-huudredth of a millimeter (the 

 12,500th of an inch) ia thickness. A contraction, then, of the crust which 

 would shorten the radius of the globe a millimeter {-^g iuch) would be 

 sufficient to produce five hundred powerful emissions of lava, and aeon- 

 traction of three centimeters (1.18 inch) would be enough to furnish lava 

 for all the eruptions history has recorded for three thousand years.* 



As to the intimate relation which exists often between earthquakes 

 and volcanic eruptions, there are so many proofs of it that even the 

 common mind has learned to recognize their connection. Thus when 

 the Italian peasant sees Vesuvius and Etna smoking and discharging 

 vapors, he has no fear for his security. But when these natural pro- 

 cesses cease to give j)assage to the gas and the vapors which are con- 

 tinually disengaged from the interior of the globe ; when calm succeeds 

 to the feeble detonations which accompany these manifestations, then 

 he is filled with anxiety and dread, for he knows that a violent eruption 

 will be the consequence of this apparent repose, and that earthquakes 

 will ravage vast extents of the surrounding country. Let us cite some 

 examples of this relation between the two phenomena. Humboldt relates 

 that the volcano Paola, situated to the north of Quito, vomited thick, 

 black smoke at the commencement of the year 1797. On the 4th of 

 February of the same year the smoke suddenly ceased, but precisely at 

 the same time occurred, fourteen miles more to the south, the terrible 

 earthquake of Kiobamba, one of tlie most violent which had ever devas- 

 tated the plateau of Quito. Scarcely was this over than the western 

 Antilles were troubled by an earthquake which lasted six months, and 

 ceased on the 27th of September with the eruption of the volcano of 

 Guadeloupe, until then inactive. When the eruption of this volcano 

 ceased, new shocks were felt in South America, which ended only with 

 the destruction of Cumana, on the 14th of December of the same year. 



Volcanic openings, or mouths, are not irregularly distributed over the 

 surface of the globe ; they are most frequently parallel to the principal 

 lines of fracture of the crust, and then they form part of mountain- 

 chains. They are rarely isolated. It is worthy of remark there are few 

 volcanic centers still active which are at a great distance from the sea. 

 What was said of earthquakes may also be said of volcanoes : they are 

 more frequent in maritime districts, for wherever the sea has retired 

 during the geological periods, the volcanoes then active have ceased to 

 be so. Witness the extinct volcanoes of Auvergne, of the Ehine, of 

 Transylvania, of Bosnia, and other volcanic cones of the Andes, of 

 Abyssinia, &c. Still we have no reason for refusing the testimony of 

 Humboldt as to the existence of volcanoes in Central Asia. "If active 

 volcanoes," says this savan, "are almost always in the neighborhood of 

 the coast, this is due less to the influence of the water of the sea, than 

 to the facility with which the crust of the globe is ruptured, at a place 

 where the declivity of the continents toward the sea, in carrying off the 



*Pouchet, Z/'fJ/jaers, note 113, p. 144. 



