wn as hard as ice,--that is, makes glaciers of them. I am
going to be on the look-out for a glacier, for you will have a good
chance to see them in this region. At the same time, during these glacial
periods, the astronomer could explain how it is that the temperature is
from ten to twenty degrees colder in both winter and summer than it is
now, so that helps the ice to accumulate. Then the glacier, flowing
slowly, slowly, (a river of ice), down the mountainsides, carries with it
quantities of the underlying rocks, till it reaches a lower level where
the ice melts and it becomes a river and carries those rocks and soil to
the sea. That way, the mountains are gradually worn down to sea level and
the whole cycle is ready to start over again."

"I see," said the ranch boy. "How long ago did you say the last glacier
period came?"

"Probably not since the time of the first men,--perhaps 30,000 years ago."

"And those glacial deposits you showed us to-day are 30,000 years old?"
the boy breathed.

"Yes, and the deposits from the glacial period before that are older
still,--a souvenir from the age of reptile dominance."

"Then when did the other ice ages come? Did you say there were five?"

"I did, but only four great ones. There were two away back in the age of
invertebrates."

"Then has the climate been the same since the last ice age?"

"Not at all. The change is gradual, and geologists naturally conclude
that some time we will have another ice age. We'll hope man has found a
better way to keep warm by that time. Our climate, with all its ups and
downs, is little by little, through the centuries, growing colder!"

"And how do you know about all these ups and downs of climate?"
challenged Long Lester.

"Why, for one thing,--we don't have to read it all from the rocks,--there
is a plain story in the rings of growth in the Big Trees. Don't you
remember those cut stumps, and the thousands of rings we counted, one for
a year? And some were wider than others, because in those years there had
been more rainfall."

"Well, I never!" was all the old prospector could articulate, as all
hands once more called it a day.

Next day Ace searched in concentric circles, but without finding a trace
of Mexicans, or, indeed, of any one.

The next night found the little party encamped an eight hours' hike up
the side of another glacial-polished slope. The trail,--that is to say
the way they picked to go,--led first to the upper end