Perhaps he was chasing a mirage.

He was on the divide of his life.

On one side were the mountains, the camps, the cattle, the wild animals--on the other the plains, the cities, and Mary.
The thought of Mary went deep.

It took hold of the foundations of his thinking and decided him.

Shuddering with the pain and despair of his love he lifted rein and rode down into the deep shadow of the long canon through which roared the swift waters of the North Fork on their long journey to the east and south.