W. R. Jillson — Recent Yolcanic Eruption. 69 



Art. YI. — New Evidence of a Recent Yolcanic Eruption 

 on Mt. St. Helens^ Washington; by Willard Eouse 

 Jillson. 



Every intellis^ent person returning from the wooded foot- 

 liills or snow-capped summit of Mt. St. Helens, Washington 

 (9671 feet — U.S.G-.S.) brings back stories of recent volcanic 

 action. The tourist, though he keep to the well-beaten, 

 government trails, sees the evidence on every side, and if he 

 will but listen, may still hear from the lips of a few old pio- 

 neering guides very interesting, though perfectly unreliable 

 reports of "the last eruption of the mountain." 



During the summer of 1915 the writer spent several weeks 

 examining the geology and physiography of the Mt. St. Helens 

 region, making the climb to the top on August 5th. Many 

 recent flows were noted at elevations of 6000 and 7000 feet, 

 but by far the greater masses of extrusive material lie below 

 4500 feet. In September while investigating the surficial 

 character of the great recent flow on the southwest side of Mt. 

 St. Helens, between Big and Cougar Creeks, the block of lava 

 shown in the accompanying illustration (flg. 1) was collected. 

 The specimen, which is now in the Yale IJniversity Geologi- 

 cal Collections of Peabody Museum, in JSTew Haven, was taken 

 from the surface of the flow at the end of a spur of old meta- 

 morphic rock which rises out of the floor of the flood, like an 

 island in the sea. The spur is, in effect, the lower end of the 

 sharp divide between two old lava-filled valleys. At this 

 point two long lava ribbons stretching down the side of Mt. St. 

 Helens unite before cascading into the bottom of the Lewis 

 River Canyon two hundred feet below. 



In spreading out laterally at the lower end of this spur to 

 join each other, the flows evidently became slower, and formed, 

 if the comparison may be allowed, a " lava back-water." From 

 the standpoint of the physical principle involved, the condi- 

 tion found here must have been essentially the same as that 

 which is seen to be operative at the foot of any stream island. 

 The slackening of the lava flow is obvious, for the surface con- 

 sists of a series of roughly outlined terraces leading down from 

 both main flows into a considerable depression which finds its 

 greatest depth close to the spur-end. As far as the preserva- 

 tion of the tree moulds or casts is concerned, this slowing up 

 and thinning out of the lava has been a fortunate thing. In 

 places where the lava is much thicker and the flows were more 

 rapid and more powerful, very few traces of tree casts are to 

 be found. At the place where the specimen shown in the 

 illustration was procured, are to be found the lava casts of a 



