502 R. M. Deeley—Plasticity of Rocks. 
have to be faced to keep the line in good condition during the winter. 
In many places the line passes over areas of waterlogged ground. 
During the winter the cold penetrates the earth and freezes this 
water to depths of 10 feet or more. The ice thus formed expands, 
raises the earth into domes and makes the road bed very irregular. 
It is the business of the engineer to see that these irregularities are 
made good by packing up the line, a troublesome task with the 
temperature many degrees below zero F. 
The ‘Canadian shield’ consists of a base of granite or gneiss rising 
into numerous domes which are somewhat irregularly scattered over 
the area. Upon them rest the Keewatin and newer rocks. After the 
deposition of the Keewatin Series the land rose and formed a great 
mountain system, which was afterwards almost completely destroyed 
by denudation. 
The rising of this mountain system seems to have been due to the 
heating up of portions of what are now known as the Laurentian 
granites, rather than to crushing or folding. We must regard the 
mountain ranges as floating upon the earth’s crust, their lightness 
being due either to their high temperature bases or small relative 
density. Crushing and folding, apart from the heating that may 
result from it, do not seem competent to alone form high mountain 
systems; for the folds would not cause a great rise of the surface— 
the mass would sink as do the sediments deposited upon a sea-bottom. 
It seems most reasonable to regard mountain ranges as being due to 
the high temperature of the deep-seated rocks causing an uplift by 
expansion, just as the railway track is lifted by the freezing of the 
water beneath. If this view be the correct one, then it follows that 
the disappearance of the mountain range must have been due to the 
cooling down of the Laurentian rocks beneath; for otherwise the 
mountain peaks would have risen as fast as they were denuded. 
Professor Coleman gives a very interesting explanation of the 
reason for the formation of the dome-shaped masses of granite and 
gneiss upon which the Keewatin rocks rest. He suggests that the 
lower rocks became plastic, that the Keewatin above was unequally 
heavy, and that the granite rose in domes where the lightest loads 
were situated. 
It must be admitted that the formation of great depressed areas, 
such as that of Lake Superior, is a difficulty. Here it is considered 
that the granite below became so fluid that it was ejected at the 
edges and allowed the area from beneath which it came to sink bodily. 
But if such be the case, how is it that this portion of the earth’s crust 
did not rise again? It may be that to some extent the explanation is 
that the deeper portions of the earth’s crust are plastic, not viscous. 
Plastic materials do not flow unless the stress exceeds some particular 
limit. A mountain range may therefore be stable although its height 
may be hundreds of feet greater or less than is warranted by the 
density of the crust at the spot. 
Professor Coleman recognizes that the weight of the ice which once 
rested on the ‘Canadian shield’ depressed it, and that since the ice 
melted away there has been arise of the area. He says these sinkings 
and risings must be accomplished by plastic flow outwards from 
