VOLCANOES. 233 



In like manner, the putrid fever, which raged seven yeais pre- 

 viously in the mountain town of Ibarra, north of Quito, was 

 ascribed to the ejection of fish from the volcano of Imbaburu.* 



Water and mud, which flow not from the crater itself, but 

 tirom the hollows in the trachytic mass of the mountain, can 

 not, strictly speaking, be classed among volcanic phenomena. 

 They are only indirectly connected with the volcanic activity 

 of the mountain, resembling, in that respect, the singular me- 

 teorological process which I have designated in my earlier writ- 

 ings by the term of volcanic storm. The hot stream which 

 rises from the crater during the eruption, and spreads itself in 

 the atmosphere, condenses into a cloud, and surrounds the col- 

 umn of fire and cinders which rises to an altitude of many 

 thousand feet. The sudden condensation of the vapors, and. 

 as Gay-Lussac has shown, the formation of a cloud of enor- 

 mous extent, increase the electric tension. Forked lightning 

 flashes from the column of cinders, and it is then easy to dis- 

 tinguish (as at the close of the eruption of Mount Vesuvius, in 

 the latter end of October, 1822) the rolling thunder of the vol- 

 canic storm from the detonations in the interior of the mount- 

 ain. The flashes of lightning that darted from the volcanic 

 cloud of steam, as we learn from Olafsen's report, killed eleven 

 horses and two men, on the eruption of the volcano of Katla- 

 gia, in Iceland, on the 17th of October, 1755. 



Having thus delineated the structure and dynamic activity 

 of volcanoes, it now remains tor us to throw a glance at the 

 differences existing in their material products. The subterra- 

 nean forces sever old combinations of matter in order to pro- 

 duce new ones, and they also continue to act upon matter as 

 long as it is in a state of liquefaction from heat, and capable 

 of being displaced. The greater or less pressure under which 

 merely softened or wholly liquid fluids are solidified, appears to 

 constitute the main difference in the formation of Plutonic and 

 volcanic rocks. The mineral mass which flows in narrow, 

 elongated streams from a volcanic opening (an earth-spring), 

 is called lava. Where many such currents meet and are ar- 

 rested in their course, they expand in width, filling large ba- 

 sins, in which they become solidified in superimposed strata. 

 These few sentences describe the general character of the prod- 

 ucts of volcanic activity. 



* [It would appear, as there is no doubt that these fishes proceed from 

 the mountain itself, that there must be large lakes in the interior, which 

 in ordinary seasons are out of the immediate influence of the volcanio 

 action See Daubeney, op. cit., p. 488, 497.]--rr. 



