down to a level and slow up; then the sand and gravel
it was carrying would settle, and the stream stop its digging. Again, if
the stream-cut valley theory is correct, why isn't every Sierra canyon a
Yosemite?

Let us look for the answer in the Sierra's history.

The present Sierra Nevada is not the first mountain chain upon its site.
The granite which underlay the folds of the first Sierra are still
disclosed in the walls of the Yosemite Valley. The granites which
underlay the second and modern Sierra are seen in the towering heights
of the crest.

Once these mountains overran a large part of our present far west. They
formed a level and very broad and high plateau; or, more accurately,
they tended to form such a plateau, but never quite succeeded, because
its central section kept caving and sinking in some of its parts as fast
as it lifted in others. Finally, in the course, perhaps, of some
millions of years, the entire central section settled several thousand
feet lower than its eastern and western edges; these edges it left
standing steep and high. This sunken part is the Great Basin of to-day.
The remaining eastern edge is the Wasatch Mountains; the remaining
western edge is the Sierra. That is why the Sierra's eastern front rises
so precipitously from the deserts of the Great Basin, while its western
side slopes gradually toward the Pacific.

But other crust changes accompanied the sinking of the Great Basin. The
principal one was the rise, in a series of upward movements, of the
remaining crest of the Sierra. These movements may have corresponded
with the sinkings of the Great Basin; both were due to tremendous
internal readjustments. And of course, whenever the Sierra crest lifted,
it tilted more sharply the whole granite block of which it was the
eastern edge. These successive tiltings are what kept the Merced and
Tenaya channels always so steeply inclined that, for millions of years,
the streams remained torrents swift enough to keep on sandpapering their
beds.

The first of these tiltings occurred in that far age which geologists
call the Cretaceous. It was inconsiderable, but enough to hasten the
speed of the streams and establish general outlines for all time. About
the middle of the Tertiary Period volcanic eruptions changed all things.
Nearly all the valleys except the Yosemite became filled with lava. Even
the crest of the range was buried a thousand feet in one place. This was
followed by a rise o