MODERN VIEWS OF DENUDATION. 455 
advance than to the great expounder of the principles of 
geology,, Sir C. Lyell. 
The next step was the recognition of ice as a mighty agent 
of denudation. 
The snows on the higher elevations of some mountain chains 
descend into the valleys in the form of glaciers, which in their 
course carry downwards the fragments of rock which fall on 
their surfaces from the heights. The glacier also scores the 
sides and bottom of the valley along which it moves, wearing 
down the rocks into mud, with which the water which issues 
from the glacier is always highly charged. In this manner, 
large quantities of matter are constantly being carried from 
the hills to the valleys, and deposited either in the deltas of 
the rivers or carried out to sea. 
A further extension of the theory of glacial denudation has 
recently been propounded by Professor Bamsay, who main- 
tains, with the aid of a strong array of illustrations, that the 
lakes which are so frequently found both amongst and around 
mountain chains, are due to the scooping power of glaciers, 
either extinct or recent. This view he grounds chiefly on 
the fact that these lakes lie in true “ rock-basins,” and that 
there is no agent in nature capable of hollowing out such 
basins except the ice of glaciers. 
It might be supposed that the effects of such an agent as 
glacial ice in the process of wearing away the solid rocks 
must be comparatively unimportant; but it must be recol- 
lected, that in the glacial period, immediately preceding the 
creation of man, snow and ice had a much wider domain over 
the whole of the northern hemisphere than at present, and 
must have exerted a corresponding effect in the denudation of 
the surface. That remarkable deposit which covers so large 
an area of North America and Europe, called the Boulder Clay, 
or Northern Drift, is to a large extent made up of materials, 
derived originally from the morains, or detritus, brought down 
by glaciers. Accordingly, we may well believe that the amount 
of wear and tear to which the higher elevations must have 
been subjected during the Glacial period was incalculably great, 
and that amongst the agents of waste not the least im- 
portant has been glacial ice. 
When we survey the present outline of the solid strata all 
over the globe, and observe how, throughout thousands of 
feet in thickness, the strata rise to the surface and are suddenly 
truncated, or cut off like the leaves of the books on the shelves 
of a library ; when, in tracing the margin of such a great forma- 
tion as the chalk, we find the beds, more than a thousand 
feet in thickness, broken off along a steep escarpment; or 
when, as in Wales, the contorted beds of slate are planed 
