1898-99. | On some Geological Agents. 25 
falls to or below zero. This work is to be seen on the moun- 
tain side in the rock detached and thrown down, in quarries, 
railway cuttings, the steep banks of rivers and streams, and 
in the stones, gravel, &c., carried down stream by surface and 
ground ice. 
Ice long, long ago had a great deal to do with the con- 
figuration and landscape of our own county. Our rounded 
hills and hillocks, our deep glens and valleys, our beds of 
boulder-clay, and our scratched and scored rocks, all remind 
us of the action of ice. Although its work is so conspicuous 
over all the land, the cause of this Ice Age is still an unsolved 
problem. The boulder-clay or till is another, with its clay- 
beds of sandstones, angular and rounded, small and great, and 
scattered indiscriminately through the moss. Much has been 
written on the subject, but no theory that has been proposed 
gives a satisfactory explanation of this deposit. 
Ice has been credited by some geologists with excavating 
our Scottish lochs and English lakes, with Geneva and Superior 
thrown into the bargain. But it has not yet been proved to 
my mind that the ice, for instance from the surrounding hill, 
excavated Loch Tay, some 15 miles long, from half a mile to 
one mile broad, and from 15 to 100 fathoms deep. If a man 
has to cut a bar of steel, he requires tools—files as hard or 
harder than the bar he is to cut. He also requires food to 
give him the necessary power. After the same manner, if a 
glacier is to excavate Loch Tay or any similar rock basin, it 
must have tools—stones or gravel, at least as hard as the rock 
it is to operate on—and gravity as the equivalent of the man’s 
food. Whether the hills surrounding Loch Tay—after having 
been subject to denudation for ages before the Glacial Period 
—could supply the necessary tools and gravitation is what I 
will not affirm. 
I have here a few specimens of undoubted ice-work. One 
is broken from a boulder weighing about five tons, excavated 
out of a clay bank about 20 feet thick. It is as smooth as if 
a planing-machine had passed over it. Another is from the 
Braid Hills, with the characteristic strie; and a third from a 
bank of boulder-clay in the Pentlands, about 1100 feet above 
sea-level. 
Sand is such a common material that we seldom give it a 
