June, 1920] The Great Magcik Landslide 337 



Of the immediate cause which set off the fall, one is left to 

 speculation. Undoubtedly there were earthquakes in connection 

 with the eruption, but they do not seem to have been partic- 

 ularly violent. It is not impossible that there may have been a 

 further uptilting of the strata by the injection of new magma 

 below. Unstable equilibrium from such a cause would be the 

 simplest, if not the only, explanation of the continual succession 

 of avalanches which have coursed almost without intermission 

 down the slopes of Falling and Noisy Mountains since their 

 discovery. It is interesting further to note that while the 

 cliff at the head of the Mageik Landslide was quiet during 

 our observations in 1917, it was subject to frequent, heavy 

 rock falls in August, 1919. There is, however, little if any 

 independent evidence of recent uplift except such as is furnished 

 by the actively falling cliffs themselves. 



DIMENSIONS OF THE SLIDE. 



The top of the cliff from which the rock started stands at an 

 elevation of about 3,000 feet. The height of its present perpen- 

 dicular face is some 750 feet. The width of the area that fell 

 away is about 2,000 feet. We have no means of knowing how 

 much further down the valley it may have projected before the 

 fall. As it stands now, since the fall, its face is a great concave 

 cirque, evidently gouged out of the face of the mountain. Its 

 bright new surface of unweathered columnar lava contrasts 

 strikingly with the duller cliffs round about. 



The -cliff from which the rock started stands on a branch to 

 one side of the main valley. From this point it swept down half 

 a mile of steep slope and then across the flat valley for nearly 

 two miles to the opposite mountain side. Then, deflected partly 

 by the mountain wall and partly by an obstructing spur part 

 way across, it turned a 60° angle and continued down the valley, 

 in a broad flat sheet a mile wide, more than two miles further. 

 Beyond this point a relatively minute tongue continued on 

 in the bed of a stream for another mile before coming to rest. 



The total length from the cliff to the extreme tip is about 

 five and a half miles, divided as follows: Upper steep portion 

 about half a mile, descending about a thousand feet; middle 

 section about one and three-quarters miles, descending 400 to 

 500 feet to an altitude of about 800 feet ; broad lower end about 



