While these collections were one of Powell's objects, others were the examination of the geology, and particularly the solution of the greatest remaining geographical problem of the United States, the canyons of the Green and Colorado rivers.

The Green, as has been explained in preceding pages, was known as far as the Uinta Mountains, and here and there at widely separated points on down to about Gunnison Valley.

But there were long gaps, and below Gunnison Crossing as far as the Grand Wash the knowledge of the canyons as already pointed out was vague in the extreme.

The altitude of Green River Station, Wyoming, was known to be about six thousand feet above sea level, and that of the mouth of the Virgen less than one thousand.

How the river made up this difference was not understood and this problem was what Powell now confronted.