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portant events in the geological history of the Pacific slope, and 
to show the relation of these events to the mountain system of the 
country and to the distribution of ores. It is not needful to insist 
upon the importance and economic interest of the latter subject. 
The mountain system will be found to be intimately connected 
with the distribution of ores, and to possess a further interest from 
the fact that it determines the climatic conditions of the region. 
There are numerous and strong reasons for supposing that the 
earth was once a fluid mass of melted minerals. Geological his¬ 
tory begins when the surface of this mass became solid. This 
was of course long since, how long it is impossible to tell with 
accuracy, but the best estimate seems to be that of Sir William 
Thomson, the famous physicist who, from certain physical data, 
arrives at the conclusion that the most probable length of the 
geological period is somewhat over one hundred millions of years. 
During this time the most various changes have occurred. The 
water and the carbon of the earth were originally portions of the 
heated atmosphere. When the atmosphere was sufficiently 
cooled, seas formed and degradation of the land surfaces began. 
Every shower carries a portion of earth into the sea and reduces 
the elevation of the land. This is erosion. There are other 
causes at work which tend to alter the configuration of the globe. 
There is a continual tendency of the land and the sea bottoms to 
rise or sink, and observations justify the conclusion that the land 
is not really more stable than the surface of the sea. All portions 
of the land are probably rising or falling or bending at every 
moment, but of course the movements are indefinitely slower 
than those of the sea. The causes of these movements are very 
imperfectly understood, but their existence is unquestionable. It 
is well known to engineers that even a mass weighing a few tons 
of a metal like bronze cannot be kept accurately in a given shape, 
but undergoes a slight deformation in consequence of its own 
weight. It is easy then to understand that even a mass of steel of 
the size of the earth may slightly and slowly change its shape. 
In the case of the earth, deformation is assisted by deep cracks 
or partings in the rocks, and when there is relative motion on the 
two sides of such a crack, we say that a “fault” is produced. 
When in the course of such a movement, which usually pro- 
