614 THE POPULAR SCIENCE MONTHLY. 



comes transformed into a granular, opaque, bulbous ice, called neve ice, 

 which at last changes into the compact blue ice of the glacier proper. 



During the summer, when the snow-fall is at its minimum, the sur- 

 face of the snow in the high regions becomes covered with dirt and 

 sand, which the next winter covers with another snow-sheet. In 

 summer these layers of snow, from a partial melting and subsequent 

 freezing, become changed to ice on their surfaces, so that we have 

 three kinds of deposits — beds of snow, sheets of dust, and layers of 

 ice. As the whole is pushed down into the valley, these layers tend 

 to assume a vertical position from the bottom of the mass, moving 

 faster than the top ; and, the snow in summer melting from the sur- 

 face as far as the snow-line, the edges of the layers are found passing 

 transversely across the glacier. The middle of the glacier being 

 deeper, moves faster than the sides, and, the lower layers advancing 

 more rapidly than the upper ones, the strata become curved for- 

 ward, the lower layers being more sharply arched. The arch thus 

 becomes the measure of the rate of movement in the different parts of 

 the glacier. From this it will be seen that Agassiz dissents from the 

 theory of Tyndall, which represents the stratified lines as due to ice- 

 cascades, or breakages of the glacier in passing over sharp angles. 



All glaciers exhibit numerous blue bands, which are parallel to the 

 planes of stratification, and are formed by thawing and freezing, and 

 by the vertical pressure of snow in the nevk Moreover, there are found 

 certain veined structures of the ice which appear to be bands of in- 

 filtration, and intersect the planes of stratification at every possible 

 angle. As they are most numerous at the sides of the glacier, it is 

 probable that Tyndall's theory of internal liquefaction of ice by press- 

 ure may account for them. 



In the progress of the glacier, its rate of movement is not uniform, 

 the differences between the centre and the sides being about as ten to 

 one. Neither is the motion uniform along the axis ; the advance being 

 greatest about half-way down the region of the nev'e, and diminishing 

 in rapidity both above and below. Agassiz found that it was from 

 20 to 50 feet per year in the higher portions, about 250 feet in the 

 n'ev'e, and diminishing again lower down. 



The causes of the movement of the glacier are several. The weight 

 of the glacier alone is not sufficient to propel it, as in this case the 

 greatest movement would be in the winter, which is not the case. The 

 principal agent is the infiltration of water, which is greatest when the 

 winter snows are melting. The granular snow of the neve incloses nu- 

 merous particles of air, which, when the snow is compressed into ice, 

 form a net-work of capillary fissures that serve as canals of infiltration, 

 the water in which, freezing, aids in propelling the glacier. Added to 

 this may be such other causes as the weight of the mass, the pressure of 

 accumulated snow above, the weight of infiltrated water, and the soft- 

 ening of the ice by water, and a consequent sliding along the surface. 



