ent dust around the whole world. The presence of the
dust caused brilliant sunsets second only to those due to Krakatoa
in 1883. It also cut off so much sunlight that the effect was felt in
measurements made by the Smithsonian Institution in the French provinces
of North Africa. In earlier times, throughout the length of the
cordillera great masses of volcanic material were poured out to form
high plateaus like those of southern Mexico or of the Columbia River in
Oregon. In Utah some of these have been lifted up so that heavy caps of
lava now form isolated sheets topping lofty plateaus. There the lowland
shepherds drive their sheep in summer and live in absolute isolation for
months at a time. There, as everywhere, the cordillera bears the marks
of mountains in the making, while the mountains of eastern America bear
the marks of those that were made when the world was young.

The geysers and hot springs of the Yellowstone are another proof of
recent volcanic activity. They owe their existence to hot rocks which
lie only a little way below the surface and which not long ago were
molten lava. The terraces and platforms built by the geysers are another
evidence that the cordillera is a region where the surface of the earth
is still being shaped into new forms by forces acting from within. The
physical features of the country are still in process of construction.

In spite of the importance of the constructive forces which are still
building up the mountains, much of the finest scenery of the cordillera
is due to the destructive forces of erosion. The majestic Columbia
Canyon, like others of its kind, is the work of running water. Glaciers
also have done their part. During the glacial period the forces which
control the paths of storms did not give to the cordillera region such
an abundance of snow as was sifted down upon Laurentia. Therefore no
such huge continental glaciers have flowed out over millions of square
miles of lower country. Nevertheless among the mountains themselves the
ice gouged and scraped and smoothed and at its lower edges deposited
great moraines. Its work today makes the cliffs and falls of the
Yosemite one of the world's most famous bits of scenery. This scenery
is young and its beauty will pass in a short time as geology counts the
years, for in natural scenery as in human life it is youth that makes
beauty. The canyons, waterfalls, and geysers of the cordillera share
their youth with the lakes, wat