304 rill': I'oruLAR science moxtiily 



An oxaii.'iiud ion of the dust from the vok-anic eiuiition of -^It. Las!-'en 

 shows it to be made up of fine dust aud broken fragments of an acid volcanic 

 rock which has been shattered to pieces by a violent explosion. Under the micro- 

 scope there are to be seen many small angular fragments of quartz, pieces of 

 triclinic feldspar showing twinning structure, perhajis oligoclase in compo- 

 sition frayed sections of brown biotite and grains of magnetite. The original 

 rock could not have been more basic than a dacite and the presence of so much 

 quartz rather suggests a rhyolite. The dust is not an ash in the sense of being 

 a fine residual ]uoduct of a cinder and there is no evidence of its having come 

 from the cooling of a molten mass. The original rock seems from the appearance 

 of the largest fragments to have been a volcanic tuff formed at some previous 

 activity of the volcano, and the late eruption has simply blown this tuff to dust. 



The erui)tions of Mt. Lassen while volcanic in their general classification 

 are in the same category as geyser eruptions the difference existing mainly in the 

 fact that the explosions of pent-up steam are so violent as to shatter and throw- 

 rock debris in the form of boulders aud dust. It is a question whether the ex- 

 plosions are very deep seated. 



Some of the uuid from the locality i^ of the same nature as the dust aud 

 probably formed from it. 



Xiiiiicroiis in(|iii]'ios liavu eoiue to tlie writer as to whether the eru[)- 

 tious of Lassen Peak are to he considered as truly volcanic, and I'ro- 

 fessor Eakh;:" indirectly raises the same jioint. 'J'liis is naturally a (|ues- 

 tion of definition merely. A volcano is primarily an opening in the 

 ground Itoiu which the internal forces of the earth project various 

 materials, mollcu j'ock l)eiiig an essential ])ro(luct al some period in the 

 history of the volcano. 



Many of the type examples of volcanic eruptions given in standard 

 college text-books are, however, of the explosive type, in which no molten 

 lava is ejected. The noted eruption of Kandai-San in Japan, on dtdy 

 1-5, 1888, is an instance. This old volcanic cone, nearly 180 miles from 

 Yokohama, had heen without sign of life for a thousand vears of re- 

 corded history, yet wdth only a few minutes of warning consisting of 

 rumblings and moderate earthquake shocks the entire top of the moun- 

 tain was l)lown away in some fifteen to twenty explosions lasting less 

 than a half hour. There was no fresh lava or pumice thrown out. Ash 

 and steam were projected upward about 4,000 feet, but the inain foi'ce 

 of the exj)losion was nearly horizontal, canying destruction in a noi'th- 

 e]'ly direction for about four miles. The cjuantity of material )»lown 

 away has been estimated at one third of a cubic mile. 



In the case of Lassen Peak the period of quiescence had jU'obably 

 been greater than a thousand 3'ears, judging from the effect of erosion 

 on the old cone. The force of the steam explosions to date has been 

 distributed through six months, yet the height of the ash-laden column 

 has several tinu's reached two miles above the mountains. Had the 

 steam been contincd more effectively in Lassen and the force, instead 



