730 



SCIENCE 



[N. S. Vol. XLIII. No. 1117 



of the old crater rim and finally after more 

 than 150 eruptions it attained near the end 

 of March, 1915, a size of about 700 to 1,000 

 feet. 



Pig. 2. Sketch contour map of the smnmit of 

 Lassen Peak by J. M. Howells, June 16, 1914, 

 upon which has been drawn the outline of the old 

 crater rim as well as the outline within it of the 

 enlarging new crater June 14, 1914, and March 

 23, 1915. The new crater at first lengthened east 

 and west and later widened north and south, in 

 both cases along joint planes, untU its dimen- 

 sions were 1,000' x 700'. Then the explosive erup- 

 tions gradually became effusive. The lava in the 

 throat of the volcano gradually upheaved during 

 April and May until it filled the old crater and 

 overflowed through the lowest notch and down 

 the west slope about 1,000 feet. The hot blast 

 that devastated Hat Creek escaped from beneath 

 the lava lid at the head of Lost Creek. 



Ejecta accumulated on the rim of the new 

 crater to a depth of thirty to forty feet. 

 The largest stone ejected was fifteen feet 

 in diameter and weighed about sixty-three 

 tons. Small stones were thrown as much 

 as a mile from the crater, but beyond two 

 miles from the crater scarcely more than a 

 trace of dust could be noticed except to 

 the northeast, the direction of the strong- 

 est winds and that taken by the great blast 

 of the eruption May 22, 1915. 



By far the greatest eruptions that have 

 occurred at Lassen Peak since its present 

 activity began are those of the night of 

 May 19 and the afternoon of May 22, 1915. 



The first great result was the extrusion of 

 new lava and the formation of a lava lid 

 which culminated in the second great fea- 

 ture, the devastation of the Lost Creek and 

 Hat Creek country by a horizontal blast of 

 hot gas. 



About the end of March, 1915, the old 

 crater having been thoroughly cleaned out 

 by explosive eruptions and the superincum- 

 bent load largely removed from the magma, 

 it began to rise in the volcanic conduit and 

 initiated the second stage, the efi'usive stage, 

 of the volcanic activity. The hot magma 

 apparently more or less viscous in the vol- 

 canic conduit, was forced upward by pres- 

 sure of magma or gas from beneath and was 

 gradually upheaved, with great escape of 

 steam, until it reached the surface as new 

 lava, and as a lava table filled not only the 

 new, but also the old crater so as to form a 

 lid on the volcano. The lava overflowing 

 from the edge of the lid through a notch 

 in the old rim, as shown in Fig. 2, passed 

 down the west slope of Lassen Peak about a 

 thousand feet. 



On the night of May 19 and especially 

 also on the afternoon of May 22, 1915, the 

 eruptions were violent. A mushroom- 

 shaped cloud was hurled to the height of 

 about four miles above the summit of the 

 mountain and afforded a magnificent spec- 

 tacle as seen from the Sacramento Valley. 

 At night'' flashes of light from the moun- 

 tain summit, flying rocket-like bodies and 

 cloud glows over the crater reflecting the 

 light from incandescent lavas below were 

 seen by many observers from various points 

 of view and appear to indicate that much 

 of the material erupted was sufficiently hot 

 to be luminous. 



3 Luminous phenomena were reported on the 

 nights of September 29, 1914, May 14, 15, 17 and 

 19, 1915, as well as June 20 and October 20 of 

 the same year. Some of the phenomena reported 

 may be explained as due to the reflected rays of 

 the setting sun, but this can not be true of all. 



