38 THE ROYAL SOCIETY OF CANADA 
The decomposition which we are now considering is that affecting 
the surface of the rock exclusively, and not that which may be caused 
at depth by the influence of juvenile waters or other deep-seated agents. 
It occurs from the surface downwards to moderate distances, and is 
usually known as weathering. 
Dry air has little influence in breaking up or altering the texture 
of rocks, except in so far asit may conduct heat or cold to them, though 
heat and cold have a powerful influence in breaking the rocks to pieces. 
In countries such as the Yukon the diurnal changes of temperature 
are often very great, for the sun at mid-day in the spring or early summer 
may shine down on the naked surface of the rock and heat it to a 
temperature of one hundred degrees F., or more, while at night the temper- 
ature may fall to zero. With such sudden changes in temperature the 
different minerals expand differently and soon loosen themselves 
one from the other and break the rock to pieces. 
An instance of this breaking down of the rock with sudden changes 
of temperature may be seen every year in the steep cliffs over-looking 
the Yukon river opposite Dawson. About the middle of November 
the river freezes over and is covered from that time until the following 
spring with a sheet of ice which extends to the foot of the cliffs. Masses 
of rock break away from the face of the cliff under the influence of 
unequal expansion caused by changes of temperature and fall on the 
ice, and before the ice melts in the spring, a talus, composed of these 
rock fragments, extends far out over it. When the ice breaks up and is 
carried away by the stream these rocks are carried away with it. 
This action doubtless goes on in summer as well as in winter; but in 
the former season a talus does not accumulate. It is quite probable 
that water and ice lodged in crevices in the rock has much to do with 
the breaking away of the rock fragments, but undoubtedly the heating 
and cooling of the rock itself is the controlling factor. 
In some countries the wind carrying a load of sand and dust has 
also a considerable influence in breaking down the surface rocks; but 
this influence is not effective to any considerable extent in the Yukon 
Territory. 
While the atmosphere thus exerts some influence in the breaking 
down of the original hard rocky surface of the earth, the principal agent 
in the formation of sands, clays and other loose deposits is water, either 
as it falls in the form of rain, or as it percolates through underground 
passages, or as it flows away in rivulets and streams. 
Pure water has little influence in rock decomposition, but when it 
falls in the form of rain it dissolves carbonic acid out of the atmosphere, 
and thiscarbonated water in its turn readily dissolves any lime carbonates 
from the rock on which it falls and carries them away with it, rendering 
