Senry Woodward — On Volcanos. 347 



Of the gaseous emanations from volcanic vents may be enumerated 

 sulphuretted hydrogen, sulphurous acid gas, hydrochloric acid, 

 nitrogen, and carbonic acid gas. 



But water, as steam, is certainly the most abundant substance 

 present, as it is also the most active agent in all volcanic out-bursts, 

 and may even be (as Mr. Scrope has supposed) the cause of the flow 

 of lava streams. We know that, in cooling, lava gives off an im- 

 mense volume of steam, and also that the vesicular cavities in lava 

 are caused by it. 



In all volcanic eruptions water appears to be present also in a 

 liquid state, and is frequently ejected during volcanic eruptions in 

 vast quantities. This may arise from three different causes : — 



(1) It may be ejected from the earth's interior with other volcanic 

 matter; or, 



(2) From the melting of snows on the sides of the crater ; or, 



(3) By the discharge of rain-water accumulated in the crater 

 during a period of repose. 



The volcano of d'Agua in Guatemala is a jDerfectly circvilar cone, 

 more than 11,000 feet high. From it descended, in 1541, a torrent of 

 water (the contents, no doubt, of a crater-lake on the summit), which 

 destroyed the old town of Guatemala, since removed to a new site. 



Living fish and alligators were observed in a crater near the lake 

 of Nicaragua, by Dr. C. Carter Blake. 



"Wherever the tropical rainfall is large, or the height of the 

 mountain condenses the snows upon its sides and crater, an eruption 

 is sure to be preceded by a " mud debacle ;" miles of country are 

 thus devastated and destroyed. The volcanos of Kilauea, Central 

 America, South America, and Java, well illustrate this. 



In 1755 jEtna sent a sudden flood of water into the Val del Bove, the 

 volume of which in one mile was estimated at 16,000,000 cubic feet. 



Of the magnitude of lava streams, that which burst forth from the 

 Skaptar Jokul in Iceland, in 1783, continued to flow for two years. 



Fig. 9. — Craterets on the lava-stream of 1783. Skaptar Jokul, Iceland. 

 It filled up the rocky beds of rivers to a depth of 600 feet by 200 

 feet wide, and spread out in the plains 12 to 15 miles wide, and 100 

 feet deep ! The two principal streams were respectively 40 and 50 

 miles in length, forming a mass surpassing Mont Blanc in magnitude. 

 The waters of springs being suddenly converted into steam formed 

 craterets over the lava stream 30 feet high. (Fig. 9.) 



