348 



Mr. Bowman on Natural Terraces 



proaches the nearest to snow, and the opposite or lower end, 

 to ice, the change being gradual and the consequence of the 

 alternate melting and freezing of the surface. Above the 

 height of about 7000 feet, the temperature of the air is seldom 

 high enough to melt the snow ; and as all ice has previously 

 been water, it is clear there can be but little of it in a solid 

 state at still greater elevations. The blocks and fragments of 

 rock that are detached from the surrounding precipices, ac- 

 cordingly sink through the mass to the bottom of the snow. 

 At less elevations, where during summer the melting process 

 is more active on the surface of the glacier, the water perco- 

 lates between the particles of the porous mass, till it reaches 

 the rocks on which it rests. Here, the temperature being- 

 lower, it is reconverted into ice, which cements together the 

 stones and gravel, and by its expansive property in passing 

 into the solid state, has a tendency to detach them from the 

 bottom. Thus a new layer or skin of ice is continually ac- 

 cumulating between the lower surface of the glacier and the 

 face of its rocky bed, which, as it thickens, acts a wedge and 

 imperceptibly keeps forcing up the whole superincumbent 

 mass, and with it the stones and gravel which it had en- 

 tangled at the bottom. But the cooperation of another agent 

 is necessary to bring them to the surface. This is the heat of 

 the sun, which during summer, especially in the day time, 

 melts the upper layers of the ice, and consequently diminishes 

 its thickness ; till at length, by this double action, the stones 

 which lay upon the bottom are lifted up till they lie exposed 

 upon the surface of the glacier. So that here, a kind of cir- 

 culation or interchange of particles takes place, as in a lake, 

 modified by circumstances ; their ascent being retarded while 

 in a state of congelation, and their descent accelerated by the 

 perpendicular cracks and fissures which everywhere intersect 

 the mass. 



This expansion from below is going on at the same time in 

 the direction of both the longitudinal and transverse axes of 

 the glacier; and as the whole mass lies in a hollow or inclined 

 trough, the power of gravity prevents it from moving in any 

 other direction than downwards. But the motion is not uni- 

 form ; the sides advance with greater rapidity than the mid- 

 dle ; so that if a row of stones were placed at equal distances 

 in a straight line across the glacier, they would soon arrange 

 themselves into a curve or arch ; those at each side being car- 

 ried down more rapidly, would form the base ; while those in 

 the middle having comparatively little motion, would be left 

 behind and form the highest part of the arch. This is owing 

 to the following cause : a slight elevation of temperature in 



