Mat 26, 1916] 



SCIENCE 



729 



The present activity of Lassen Peak, 

 though feeble as compared with its earlier 

 eruptions, is proof that it must still be 

 classed as an active volcano. 



The volcanic activity which resulted in 

 the upbuilding of Lassen Peak began near 

 the close of the Eocene. The lava flows ap- 

 pear to have been largest and most numer- 

 ous in the Miocene and Pliocene, successive 

 flows decreasing in size during the Quater- 

 nary to near extinction in recent times. 



There were long periods of interrupted 

 activity separated by long intervals of qui- 

 escence. During the active periods both 

 explosive and effusive eruptions were com- 

 mon; the one forming cinder cones and 

 sheets of volcanic agglomerate and tuff; 

 the other forming lava fields whose rugged- 

 ness was proportional to the viscosity of the 

 erupting lava. 



Lassen Peak is a volcano of large type 

 surrounded by many smaller ones of later 

 date, the whole being built up of a notable 

 variety of lavas. The oldest lavas of the 

 Lassen Peak region are of intermediate 

 chemical composition belonging to andesites. 

 The early magma yielding the erupting 

 andesitic lavas in the course of time differ- 

 entiated into two portions. On the one 

 hand it became more siliceous (salic), 

 erupting as dacite and rhyolite, and on the 

 other hand it became less siliceous (mafic) , 

 yielding basalt and quartz basalt. All vari- 

 eties are well represented in the Lassen 

 Peak region and are derived apparently 

 from the same magma. 



As the volcanic center developed the most 

 active crater migrated. The first crater was 

 in the head of Mill Creek. It was not only 

 the oldest, but also the largest crater, more 

 than a mile in diameter. Composed of an- 

 desitic lavas, it rose to a height of 9,400 feet. 

 The peak named "Brokeoff Mountain" on 

 the Forest Service maps is the most promi- 

 nent remnant of this great crater in the 

 head of MiU Creek. 



The second great crater opened on the 

 northern edge of the first and erupted 

 dacite, building up Lassen Peak to its pres- 

 ent height with a summit crater about a 

 quarter of a mile in diameter. 



The third crater, about four miles a little 

 west of north from the first, opened only a 

 few centuries ago at the northwest base of 

 Lassen Peak, and the rugged lava flows 

 from it formed Chaos Crags. 



The products of this eruption in Chaos 

 Crags are well preserved and their rela- 

 tions clearly visible. The eruption began 

 by a succession of explosions that spread a 

 thin layer of volcanic sand and dust over 

 the surrounding country and ended in the 

 extravasation of a most rugged mass of 

 dacite which, though at first glance having 

 the aspect of granite, is rich in volcanic 

 glass, generally of dark color, somewhat 

 pumiceous and full of inclusions like the 

 dacites of Lassen Peak. 



The fourth crater of Lassen Peak is the 

 new crater, active at the present time. It 

 began by a slight explosion within the old 

 crater, second of those enumerated, on 

 the summit of Lassen Peak, and is remark- 

 able for its place of outbreak, as well as its 

 low energy, the small mass of material 

 erupted and the continuity of the activity. 

 Like the eruption of a few centuries ago at 

 Chaos Crags, it had two phases, one explo- 

 sive, the other effusive. 



During the first phase the explosive erup- 

 tions were of gas carrying out with it rock 

 fragments and dust only. The size of the 

 crater increased with each eruption, as 

 shown in Fig. 2 by the outlines of the new 

 crater June 14, 1914, and March 23, 1915. 

 The second phase, which is effusive, includes 

 also an eruption of lava, which formed a lid 

 on the volcano and overflowed to the west, 

 as represented in Fig. 2. 



In the beginning the new crater was eon- 

 fined to the loose material filling the old 

 erater, but later it reached the solid rock 



