262 COMPARATIVELY MODERN DATE OF THE CHAP. XIIT. 



percolation through the porous parts of the moraine, and not 

 by a stream overflowing that barrier. Such a glacier-lake 

 Dr. Hooker actually found in existence near the head of the 

 Yangma valley in the Himalaya. It was moreover partially 

 bounded by recently formed marginal terraces or parallel 

 roads, implying changes of level in the barrier of ice and 

 moraine matter.* 



It has been sometimes objected to the hypothesis of glacier- 

 lakes, as applied to the case of Glen Roy, that the shelves 

 must have taken a very long period for their formation. Such 

 a lapse of time, it is said, might be consistent with the theory 

 of pauses or stationary periods in the rise of the land during an 

 intermittent upward movement, but it is' hardly compatible 

 with the idea of so precarious and fluctuating a barrier as a 

 mass of ice. But the reader will have seen that the perma 

 nency of level in such glacier-lakes has no necessary con 

 nection with minor changes in the height of the supposed 

 dam of ice. If a glacier descending from higher mountains 

 through a tributary glen enters the main valley in which 

 there happens to be no glacier, the river is arrested in its 

 course and a lake is formed. The dam may be constantly 

 repaired and may vary in height several hundreds of feet 

 without affecting the level of the lake, so long as the surplus 

 waters escape over a c col' or parting ridge of rock. The 

 height at which the waters remain stationary is determined 

 solely by the elevation of the ' col,' and not by the barrier 

 of ice, provided the barrier is higher than the ' col.' 



But if we embrace the theory of glacier-lakes, we must be 

 prepared to assume not only that the sea had nothing to do 

 with the original formation of the e parallel roads,' but that 

 it has never, since the disappearance of the lakes, risen in 

 any one of the glens up to the level of the lowest shelf, which 



* Hooker, Himalaya Journal, vol. i. also profited by the author's personal 

 p. 242 ; ii. pp. 119, 121, 166. I have explanations. 



