EVIDENCES OF THE GEEAT EEOSION. 
73 
structural deformations which they traverse; that the elevation of a plat- 
form across the track of a river rarely diverts it from its course, for the 
stream saws its bed into the rocks as fast as the obstacle rises. It would be 
impossible to point out a more complete illustration of these propositions 
than that which is supplied by the Colorado and its tributaries. 
We know that during the whole of Mesozoic time the watershed of the 
Colorado was submerged and that in Eocene time it was a great fresh- water 
lake. In due time this lake was drained presumably by the cutting down 
of its outlet as the country rose. In this process may be discerned tlie origin 
of its drainage system; and we are bound to infer that every river then 
existing within it ran in conformity with the surface just exposed above the 
waters. To-day we find that surface greatly deformed by displacements 
and by erosion, and the courses of the rivers to be such as they could not 
have been if these inequalities and deformations were as old as the rivers. 
It would be an endless task and very burdensome both to the writer and to 
the reader to analyze the course of the Colorado and each of its tributaries 
to show their relations to the structural features. The subject may be sum- 
marized in the single statement that they are entirely independent of the 
structural features. They run in a majority of cases against the inclinations 
of the strata and against the topographical slopes. They cut through 
mountains and plateaus ; they enter cliffs, they emerge from them ; they 
enter the lifts of monoclines, they cross faults from the upthrow to the down- 
throw. They run here obliquely up or down the structural slopes, and 
there they course along the strike. 
There is one and only one way in which we can account for the present 
positions of these drainage channels. -Confining ourselves to the Grand Canon 
district (though the same generalization holds good for the entire watershed 
of the Colorado), we shall find a consistent explanation of the drainage prob- 
lem by assuming precisely what we have deduced from the discussion of the 
stratification and displacements; i. e., depi'essing the whole Grand Canon 
platform many thousands of feet and covering it with the Mesozoic and 
Eocene beds in full volume, reducing at the same time all the faults and 
flexures until the Carboniferous becomes a smooth platform over the whole 
district. 
