al changes drove man's
ancestors down out of the trees." The boys, no less than the old
prospector, testifying their interest in the last named operation, he
continued.

"When the Alps, the Andes and the Himalayas arose, man's ancestors still
lived in trees. But high mountains hold a large part of the moisture of
the atmosphere in the form of snow and ice, and at the same time the
decreased oceanic areas offer less surface for evaporation. Not only does
that mean a drier climate, but the sun's rays pass more freely through
dry air, and the days are hotter, and the heat passing freely back
through the same dry air at night, the nights are colder. Seasons are
more extreme, and ice accumulates on the mountain tops and around the
polar region, precursor of a glacier period. The aridity decreases the
amount of forest, and the manlike tree dweller had to descend to the
ground to get his living. That necessitated the development of his hind
legs for speed, and that speed necessitated his assuming a wholly erect
posture. That in turn freed his hands, and he, or the man descended from
him, could defend himself by throwing stones at the huge beasts who then
peopled the earth. The cold winters necessitated the use of the skins of
beasts for clothing, and so on through the list. It was geological
necessity that drove man into his higher development.

"Changes of climate and environment, however, are stimulating, even
to-day. Statistics show that stormy weather actually increases people's
energy."

The next day they passed a long crack in a rock slope, which Norris felt
sure had been made by an earthquake, perhaps as recent as that of 1906,
to judge from the cleanness and newness of it. The crack was no more than
a foot or two in width, but in places eight feet deep, they estimated,
and along the Western side of it stood a fault scarp, in this case a wall
of granite bowlders of various sizes up to four or five feet in height.

"This," pronounced the geology man, "is evidently a region overlying
subterranean volcanoes, which might even yet build the range higher. I'll
bet that kind of mountain building may still be going on around here."

Again and again Norris, or even Ace, had been able to point out, in the
record of the rocks, the evidences of the two glacier periods that had
helped shape the Sierra Nevada, the earlier one much larger, and enduring
longer, as shown by the moraines (or deposits) left behind. The lower end
of a