31 
X. 
On the Cause of the Descent of Glaciers. 
By the Rev. Canon Moseley, F.E.S., Instit. Imp. Sc. Paris Corresp 
Read at the General Meeting, December 2nd, 1869. 
As you look at the hollows scooped out between mountain and mountain 
in an Alpine chain you see the surface of the snow that fills them and i s 
perfectly smooth near the top gradually and almost imperceptibly changing 
the uniform smoothness of its hollow face, until at length there is a sharpness 
of definition at its edges which marks the line of its course, and tells you 
that it has become solid ice. That ice is a glacier. It has come from under 
the snow far above the point where you first see it, and it continues winding 
far down into the valley below. Glaciers do not take their origin in the 
highest Alpine regions. It is not there that the snow chiefly falls, but on a 
belt girding them below. This wide belt is divided horizontally into an 
upper and lower part by the snow line, at a height of, from 3000 to 3300 
yards. Above that line snow always lies, and rain very rarely falls ; beneath 
the snow line the snow disappears every summer, and rains are abundant. 
It is from this belt about the snow line that the glaciers are seen emerging. 
They lie like huge slugs along the descending valley, swelling themselves 
out to fill their channels where they are wide, and thinning themselves to 
pass through the gorges and narrow places in them. They seldom come down 
to a lower level than 3400 feet. Between this level where they end and the 
snow line, 9000 feet high, where they begin, they traverse sometimes a very long 
space— lying for the most part at a low pitch. The resemblance to a huge 
mollusk, sometimes 10 miles long and more than a mile broad, is kept up in 
this that they move with a strange slow motion, not altogether unlike that 
of such an animal. The parallel will be complete if we conceive it to have 
its tail continually renewed as it withdraws it from under the snow line, and 
its head continually melted away as it thrusts it forward below the level of, 
from 3000 to 4000 feet. If we further imagine the steep sides of the valley 
through which the glacier, descends to have similar but smaller glaciers 
crawling down them to the principal glacier, we shall Understand what is 
meant by tributary or secondary glaciers, which are thus placed in regard to 
the principal ones ; having a far greater pitch or slope than they, and flowing 
into them like tributary streams to a river. The slope of a principal glacier 
is often as little as 3°, and yet it may move with a velocity of 24 inches a 
day. The slope of a tributary glacier is sometimes 50° and it does not 
