CAUSES or VOLCANIC ACTIVITY DAY 263 



feet above the summit of the mountain and blocks the size of a man'? 

 hand were thrown for 10 miles. During this culminating outbreak, 

 or just before it, the old volcano plug forming the floor of the crater 

 was raised ^M)0 feet or more to a position level with the crater rim, 

 the force behind it apparently being just insufficient to blow it com- 

 pletely off the mountain and so to expose the volcano hearth. The 

 point of greatest weakness then proved to be at the northeast side of 

 the inclosing cone, somewhat below the summit, and two devastat- 

 ing, horizontal explosions, the latter concluding the period of most 

 violent activity, found vent through this opening. Although the 

 adjacent valleys of Hat Creek and Lost Creek were stripped of all 

 vegetation for a distance of more than four miles by these horizontal 

 blasts (pi. 2. fig. 2), no fire appears to have been set by the explosions, 

 save momentarily on the face of a steep cliff directly exposed to the 

 second blast, and even this Avas confined to a small area of dead 

 leaves and twigs which supported combustion for no more than a 

 few moments. Red-hot ejecta at the summit or illuminated smoke 

 clouds at night w^ere seen only once (May 19, 1015) and then the 

 color was described as " deep red," indicating a temperature not 

 higher than 750°. Generally the ejecta which were thickly strewn 

 about the country adjacent to the crater were not warm enough to 

 melt the snow on which they fell and even when of great size they 

 broke through the snow without melting it away. In midsummer 

 likewise no case is known where the ejecta were hot enough to kindle 

 a forest fire, although a careful watch was kept by the Forest Ranger 

 Service through four summers for just such a contingency. 



Following this outburst the eruptive activity gradually subsided, 

 the year 1916 offering but a few mild explosions and 1917 only a 

 short concluding outbreak (also in May), of considerable violence 

 wdiile it lasted, but not comparable either in volume or intensity with 

 tlie great outbreak of May, 1915.'^ 



This brief outline of the character of the eruption of Lassen Peak 

 is sufficient to show that in spite of the violence of the explosions 

 no evidence of the high temperatures of Kilauea or Vesuvius ^ ap- 

 peared at the surface during the entire period of activity. There 

 were steam explosions of tremendous volume and power, but no local 

 development of heat by chemical activity, such as has been described 

 in the case of Kilauea, was detected at any time. 



In considering this case it is appropriate to emphasize the fact 

 that there has been no other eruption of this volcano since the region 



^ For further dotails and laboratory studies see Arthur L. Day and E. T. Allen, "The 

 Volcanic Activity and Hot Springe of Lassen I'eak," Carnegie Inst, of Washington, I'lih- 

 lication No. 860, 1025. 



^ For an account of activity of Vesuvius in recent years see F. A. Ferret, " The Vesu- 

 vius Eruption of 1006, Study of a Volcanic Cycle." Carnegie Inst, of Washington, Pub- 

 lication No. 339, 1924. 



