s were arter me!"

"You've been dreaming," Norris laughed, while the boys fairly rolled over
one another in their enjoyment.

Ace and Ted now made two flights daily in search of the Mexicans, or the
smoke of their cook-fire.

Next day they came to a canyon that filled the Geological Survey man with
profound enthusiasm, for, he said, it illustrated both the last glacial
period and the last period of volcanic mountain building. First they
noted that the little mountain stream had worn its torrential way through
the basalt or volcanic rock in a narrow canyon perhaps 200 feet deep. A
flow of molten basalt, accompanied by cinders, had been erupted from the
8,000-foot peak at the upper end of the canyon, and had flowed down in a
layer 200 feet thick when it hardened. It had flowed,--as the underlying
rock still showed in places,--over a lateral moraine or rock debris left
by a glacier as it flowed down that way. And from the weathered condition
of this rock debris, Norris said, it must have been a glacier, not of
the last ice age, but of the one preceding,--for of the four glacier
periods generally recognized by geologists to-day, evidences of the last
two can be seen in the Sierras.

What made this little canyon even more of a find, (from the point of
view of what he wanted to show the boys), was that on top of the volcanic
rock lay the deposit from another glacier, one that flowed in the last
ice age, as the condition of the rock debris plainly showed the expert.

The boys tucked a few rock specimens into their packs and launched an
avalanche of questions. But he made them wait till they had established
all snug for the night beside a stretch of rapids, where they could look
forward to catching trout for breakfast. Then, lighting his pipe, and
stretching his feet to the bon-fire,--for the night wind swept cool upon
them,--Norris began with Ted's question as to glaciers and volcanoes.

"During the times I spoke of last night, when the earth crust is
breaking, the molten rock and gases and water vapor in the interior of
the planet rise in the hearts of the mountain ranges, and often break
through as active volcanoes, pouring their lava and ash over the
underlying granite, and building it still higher.

"These heightened mountain ranges bring about the glacial climates. For
the snows on their cold peaks do not melt when summer comes, and
consequently they accumulate, and accumulate, till their own weight
presses them do