586 CLARENCE N. FENNER 



talus pile of considerable size has been built up by such accumula- 

 tions, but this is of insignificant magnitude in comparison with 

 the total quantity that has been lost to the mountain. Strangely 

 enough, there is little hint as to what has become of this mass of 

 rock. The mantle of ash and pumice that covers the floor of the 

 valley at the foot of the mountain spreads its smooth contours over 

 the whole surface. I think we must conclude that the great rock- 

 avalanche at Falling Mountain was one of the first events accom- 

 panying the recent outbreak of volcanic activity, and that it 

 occurred under such conditions of forcible disruption and violent 

 movement that the material was spread widely over the valley 

 floor. The subsequent deposits of ash and pumice, which here are 

 of very great thickness, smoothed out the irregularities left in the 

 surface of the transported material. The rock-falls that we now 

 observe are probably of the nature of after-effects. Remnants of 

 the fissures that were formed at the time of the original disturbances 

 now afford passages for gases and vapors from below. Along 

 these channels the andesitic wall-rock has been powerfully acted 

 upon and transformed into porous aggregates of new minerals, 

 whereby the rock rapidly loses its cohesive strength. It is not 

 surprising to find that among the new minerals tridymite is promi- 

 nent. The conditions are those under which its formation (as a 

 metastable product) is to be expected. A noticeable effect also is 

 the replacement of many of the pyroxene phenocrysts by aggregates 

 of hematite scales. 



These alterations seem to be explicable only on assumptions of 

 rather wide-reaching significance. Apparently the gases that per- 

 meate the rocks and that manifest themselves at the surface by 

 the slowly rising vapor clouds are capable of reacting with the 

 constituent minerals in such a way as to form volatile com- 

 pounds, and the porosity indicates that quantities of material 

 have actually been removed by gaseous transfer. The results of 

 similar processes are visible around the vents of many of the 

 fumaroles on the floor of the valley. Here also there is evidence 

 of the transportation of material in the gaseous medium, and we 

 observe the results of reactions induced by rapidly changing con- 

 ditions of temperature and composition as the gases approach the 



