t upon the northwest their progress was stopped by
canons and deserts.

Now we are prepared to understand why it was that the western portion
of North America remained for so long a time a mysterious and unknown
region. There were no waterways by which it could be explored,
while snow-clad mountains and deserts made access to it doubly
difficult.

By the beginning of the last century the Americans had overcome the
natural obstacles in their westward progress, and their settlements
reached as far into the wilderness as the Mississippi River. Hunters
and traders were soon pushing far beyond, spreading over the Great
Plains and up to the very base of the Rocky, or Stony Mountains, as
they were then called. The Missouri River became the great highway
into the Northwest, for the adventurers took advantage of the streams
wherever possible. Many other rivers were discovered flowing from
the western mountains, but with the exception of the Platte and
Arkansas they were generally too shallow for navigation even with
a light canoe.

Starting in the early spring from the mouth of the Missouri, the
hardy trappers sailed and paddled up the river, taking several
months to reach the head of navigation at the Great Falls. In the
autumn, when the boats were loaded with furs, it was a comparatively
easy matter to drop down the river with the current. It would have
been almost impossible to transport the loads of goods on pack-horses
across the thousand miles of prairie, where the traders would be
subject to attack from hostile Indians.

Adventurous men pushed farther and farther west through the passes
in the mountains and began trapping upon the waters which flow into
the Pacific. It had long been supposed that the Rocky Mountains
formed a barrier beyond which our country could not be extended,
and that the Pacific slope was made up of mountains and deserts
not worth securing.

The explorers showed that the Rocky Mountains were not continuous,
but consisted of partly detached ranges, and that while their eastern
fronts were indeed almost impassable for long distances, there
were places so low that it was difficult to locate the exact spot
where the waters parted to seek the Pacific Ocean and the Gulf of
Mexico. In southwestern Wyoming the continental divide, known as
the Great Divide mesa, though more than a mile above the sea, is
but a continuation of the long, gentle slope of the Great Plains.

The Rocky Mountains decrease in