IGNEOUS AGENCIES. 135 



the sea seems in many cases to be ' associated with their 

 gradual extinction. The recently extinct volcanoes on 

 the east side of the Sierra are good examples. 



Phenomena of an Eruption. In some cases, as in 

 the Hawaiian volcanoes, the floor of the crater, hardened 

 from previous eruption, becomes hot, then melts ; then 

 the melted lava rises higher and higher, until it overflows 

 and runs down the slope in one or more streams. The 

 volcanic forces being thus relieved, the melted lava again 

 sinks gradually to its former level, and hardens into a 

 floor. Thus all proceeds with but little commotion. In 

 other cases, as in the Javanese volcanoes, premonitions of 

 coming violence are observed in the form of subterranean 

 explosions attended with shakings of the earth ; then, 

 with a powerful explosion, the floor of the crater is broken 

 up, and the fragments are shot with violence, high, some- 

 times miles high, in the air ; then cinders and ashes and 

 smoke are ejected in immense volumes ; then streams of 

 lava are outpoured, perhaps alternating with explosions 

 of gas and vapor,, ejecting cinders and ashes. The ascend- 

 ing vapors are condensed, and fall as deluges of rain, 

 which, with ejected ashes, form streams of mud. In all 

 cases, if the mountain be snow-capped, the melting of 

 the snow produces floods, which are often among the 

 most disastrous features of the eruption. 



Thus there are two extreme types of eruptions, the 

 quiet and the explosive. In the one, the ejecta are mostly 

 lavas; in the other, gases, vapors, ashes, and cinders. 

 The best type of the former are the Hawaiian, of the 

 latter the Javanese volcanoes. But all grades between 

 exist. The Icelandic volcanoes belong more nearly to the 

 former type, the Mediterranean to the latter. Among 

 Mediterranean, Etna approaches more the former, and 

 Vesuvius the latter. 



Quantity of Matter Ejected. In the great eruption 

 of Tomboro, in the Island of Sumbawa (one of the Jav- 



