They were the Rocky Mountains.
For the first time that morning Uncle Prudent and Phil Evans were sensible of a certain lowness of temperature which was not due to a change in the weather, for the sun shone in superb splendor.
"It is because of the "Albatross" being higher in the air," said Phil Evans.
In fact the barometer outside the central deck-house had fallen 540 millimeters, thus indicating an elevation of about 10,000 feet above the sea.

The aeronef was at this altitude owing to the elevation of the ground.

An hour before she had been at a height of 13,000 feet, and behind her were mountains covered with perpetual snow.
There was nothing Uncle Prudent and his companion could remember which would lead them to discover where they were.

During the night the "Albatross" had made several stretches north and south at tremendous speed, and that was what had put them out of their reckoning.
After talking over several hypotheses more or less plausible they came to the conclusion that this country encircled with mountains must be the district declared by an Act of Congress in March, 1872, to be the National Park of the United States.

A strange region it was.