"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 cliffs in order to be safe. Nobody seems to know what they were afraid of, whether savage tribes, or great beasts that may have roamed this part of the country a thousand and more years ago." "And that's the bait that has drawn the old scientist here, to study it all out, and write up the history of the people who looked on this very picture so many hundreds of years back. Why, Frank, some of the cliffs they say are about a mile high! That's hard to believe, for a fact."