''ROCK GLACIERS'' OR CHRYSTOCRENES 



551 



beneath the slide, one of these springs issues from the fractured ser- 

 pentine. In the early days of the city this spring furnished the best 

 supply of water for drinking purposes available in the vicinity. 



During the summer season the water from this spring flows freely 

 from the rock and down through the talus slope at the foot of the hill, 

 but in winter, when the thermometer often drops to 50 below zero, 



Fig. 2. — Upper part of the Slide at Dawson. 



or lower, it freezes as it issues from the rock, or as it courses through 

 the loose rock of the talus heap, and imbeds the broken fragments of 

 rock in a matrix of clear, solid ice. At the same time loose pieces of 

 rock are being constantly detached from the face of the hillside and 

 dropped on this heap of rock-fragments lying below. In this w^ay the 

 heap of broken rock is being built up, and the weight of its higher 

 side is being constantly increased. On account of this increase in 

 weight, and also on account of the filling of the interstices between the 

 rock-fragments with ice, there is a constant tendency for the pile of 



