PKOCEEDINGS: GEOLOGICAL SOCIETY 405 



the flow structure in some of the breadcrusted bombs shows them to 

 have been viscous. 



Arthur L. Day: Volcanic phenomena at Lassen Peak. Since the 

 outbreak in 1914, Lassen Peak has shown four phases of activity which 

 may be called volcanic. The first phase began on May 30, 1914, with 

 a series of sharp explosions in the old summit crater, which developed 

 an opening in the scoriaceous debris at the bottom of the crater some 

 25 by 40 feet in size and perhaps 30 feet deep. These explosions were 

 followed by others with increasing violence for several weeks, until the 

 new opening reached a length of 900 feet or more. No fresh lava or 

 other evidences of heat or chemical action appeared during this phase 

 of the activity. 



On May 21, 1915, the most violent explosion thus far noted took place, 

 and was accompanied by a horizontal blast down the northeast flank 

 of the mountain (phase 2), a mud flow (phase 3) following the blast, 

 and a summit upheaval (phase 4) of much greater magnitude than 

 any which had preceded it. The lateral blast was similar in its charac- 

 ter and effects to the "Nuees Ardentes" of Mont Pelee, so accurately 

 described by Lacroix. The temperature, however, though sufficient to 

 melt the vast accumulations of snow on this flank of the mountain, 

 and thus to cause the mud flow, was nevertheless insufficient to start 

 a general conflagration of the kind which was visited upon Saint Pierre. 

 The only evidences of combustion were confined to a single small area 

 where local conditions interrupted the path of the blast and increased 

 the time of exposure to its heat. A similar blast occurred on May 23. 



That the mud flows were caused by the melting snow on the outer 

 flanks of the mountain, rather than by an outflow of mud from the 

 crater, as at first reported, is plainly established by the observation 

 that no mud is found within some 1500 feet of the summit. The devas- 

 tation caused by the mud flow was, nevertheless, of considerable mag- 

 nitude, and involved some 5 million feet of standing timber, much of 

 which was swept away, root and branch. 



The upheaval at the summit, lifting a considerable portion of the 

 bottom of the old crater, including sections of the east and west rims, 

 but not being sufficiently powerful to hurl it completely off from the 

 mountain top, produced the appearance of an area which has been 

 effectively dynamited. At the east end, where the lateral blast found 

 vent, some large boulders were split off and were carried by the mud 

 flow into the valleys below. The remaining upheaved matter fell back 

 in a wild chaos of boulders, the summit of which is now 200 feet or 

 more above the lowest point of the old crater bowl. 



A visit to this upheaved area about 4 weeks after the occurrence re- 

 vealed a few cracks, adjacent to the center of explosive activity, from 

 which hot gases were still escaping. Except for the rocks and ash 

 adjacent to these cracks, all the upheaved matter was cold. From this 

 and other surface indications, it seems impossible to conclude that any 

 fresh lava reached the surface, nor could any considerable evidence of 

 chemically active gases be found. 



