PROGRESSION OF GLACIERS. 
249 
There is still a third cause, to which the same 
result may possibly be due, and to which I shall 
refer at greater length hereafter ; but as it has 
not, like the preceding ones, been the subject of 
direct observation, it must be considered as ky- 
potlietical. The admirable experiments of Dr. 
Tyndall have shown that water may be gener- 
ated in ice by pressure, and it is therefore possi- 
ble that at a lower depth in the glacier, where 
the incumbent weight of the mass above is suffi- 
ient to produce water, the water thus accu- 
mulated may be frozen into ice-layers. But this 
depends so much upon the internal tempera- 
ture of the glacier, about which we know little 
beyond a comparatively superficial depth, that it 
cannot at present afford a sound basis even for 
conjecture. 
There are, then, in the upper snow-fields three 
kinds of horizontal deposits : the beds of snow, 
the sheets of dust, and the layers of ice, alternat- 
ing with each other. If, now, there were no mod- 
ifying circumstances to change the outline and 
surface of the glacier, — if it moved on uninter- 
ruptedly through an open valley, the lower lay- 
ers, forming the mass, getting by degrees the ad- 
vance of the upper ones, our problem would be 
simple enough. We should then have a longi- 
tudinal mass of snow, enclosed between rocky 
walls, its surface crossed by straight transverse 
