way down from beginning to end, only on one
side."

"I reckon it would, for you'd have to trace every one of these lateral
gashes up to its source, so as to cross over. And that would mean
thousands of miles to be covered."

"Gee!" exclaimed Bob, throwing up his hands as he spoke; "when you say
that, it makes a fellow have some little idea of the size of this hole.
And to think it's come just by the river eating away the soil!"

"They call that erosion," remarked Frank, who had of course posted
himself on many of these facts, during his previous visit to the canyons
of the Little Colorado. "It's been going on for untold thousands of
years; and as the river with its tributaries has gradually eaten away
the soil and rocks, it has left the grandest pictured and colored walls
ever seen in any part of this old earth."

"When that afternoon sun shines on the red rocks it makes them look
almost like blood," declared Bob. "And already I'm glad we came. I think
just now I could be happy spending months prowling around here, finding
new pictures every day."

"Then you don't blame old Uncle Felix for staying, do you?" laughed
Frank.

"Sure I don't," returned the other lad, with vehemence. "And besides,
you must remember that he had another string to his bow."

"Meaning his craze to be the fortunate man of science to unravel the
mystery that has always hung over the homes of those cliff dwellers?"
Frank went on.

"I can understand how it must appeal to a man living as Professor Felix
has all these years," mused Bob. "And think of those queer old fellows
picking out this one place of all the wide country to build their
homes."

"That was because there could be no place that offered them a tenth of
the advantages this did," Frank remarked, pointing across the wide chasm
to the towering heights that could be seen. "Think of hundreds of miles
of such cliffs to choose from! And as the softer rock was washed out by
the action of floods countless ages ago, leaving the harder in the shape
of astonishing shelves and buttes, these people took a lesson from
nature, and carved their roomy homes by following the pliable stone."

"Say," Bob exclaimed, "that makes me think of what I read about the
catacombs of Rome; how, for hundreds of miles, they run in every
direction, following the course of veins of earth in the rock, that
were selected by those who dug 'em."

"Of course," said Frank, "these people built their homes up in the