At last they decided that the stock could stand as picked out, with the exception of Walter Perkins's mustang, which went lame shortly after the boy had started off with him.

"I guess we are all right now," announced Tad, riding up to where the Professor and Jim Nance were standing. "Has either of you any suggestions to offer?"

"Hain't got no suggestions to offer to the likes of you," grumbled the guide. "Where'd you learn to ride like that?"

"Oh, I don't know. It came natural, I guess," replied Tad simply. "The others ride as well as I do."

"Then we'll be moving. I reckon you are figgering on gitting started to-day?"

"Yes, we might as well be on our way as soon as you are ready, Mr.

Nance," agreed the Professor.

"How about the pack train?" asked Tad.

"The mules are all ready," answered the guide.

The lads rode their new horses back to Flagstaff. None cared to ride in the buckboard long as there was a horse to ride. Even the Professor thought he would feel at home in the saddle once more. Nance observed that though Professor Zepplin was not the equal of the Pony Riders on horseback, yet he was a good man in the saddle. Nance was observing them all. He knew they would be together for some weeks and it was well to understand the peculiarities of each one of the party at the earliest possible moment.

Reaching town the party found that the entire equipment for the pack train had been gotten in readiness. There remained but to pack the mules and they would be ready for their start. This was done with a will, and about two o'clock in the afternoon the outfit set off over the stage road, headed for the Grand Canyon.

It was a happy party, full of song and jest and joy for that which was before them. The way led through the Coconino Park. Some three miles out they halted at the edge of a dry lake basin, in the centre of which was a great gaping hole. The Professor pointed to it inquiringly.

"There was a lake here up to a few years ago," explained Jim. "Bottom fell out and the water fell in. Ain't no bottom to it now at all"

"Then---then the water must have leaked out on the other side of the world," stammered Chunky, his eyes big with wonder.

"I reckon it must have soused a heathen Chinee," answered Nance, with a grin.

"Pity it didn't fall out the other way and souse a few guides, eh?"

questioned the fat boy, with a good-natured grimace at which Nance laughed inwardly, his shaking whiskers being the only evidence of any emotion whatever.

"Up there is Walnut Canyon," explained Jim. "Cliff dwellers lived up there some time ago."

"Yes, we met some of them down south," nodded Chunky.

"You mean we saw where they once lived long, long ago," corrected Professor Zepplin.

"Yes, we saw where they lived," agreed Stacy.

The way led on through a forest of pines, the trail underfoot being of lava, as hard and smooth as a road could be. They were gradually drawing nearer to Sunset Mountain. After a time they turned off to the right, heading straight for the mountain.

Tad rode back to the Professor to find out where they were going.

"I thought you boys might like to explore the mountain. You will find some things there well worth scientific consideration."

"Yes, sir; that will be fine."

"You know the mountain was once a great volcano."

"How long ago?" interrupted Stacy.

"A few million years or so."

"Mr. Nance must have been a boy in short trousers then," returned Stacy quizzically. The guide's whiskers bristled and stood out straight.

The road by this time had lost its hardness. The ponies' hoofs sank deep into the cinders, making progress slow for the party. They managed to get to the base of the mountain, but the mustangs were pretty well f.a.gged. The animals were turned out for the night after having been hobbled so that they could not stray far away.

"Now each of you will have to carry a pack," announced the guide.

"I will tell you what to take."

"Why, where are we going?" asked Tad.

"We are going to spend the night in the crater of the extinct volcano,"

said the Professor. "Will not that be a strange experience?"

"Hurrah for the crater!" shouted the boys.

"Speaking of volcanoes, I wish you wouldn't open your mouth so wide, Ned. It makes me dizzy. I'm afraid I'll fall in," growled Chunky.

CHAPTER IV

A NIGHT IN THE CRATER

"What, climb that mountain?" demanded Stacy.

"Surely. You are not afraid of a mountain, are you?" demanded Tad.

"I'm not afraid of---of anything, but I'm delicate, I tell yau."

"Just the same, you'll pack about fifty pounds up the side of that hill,"

jeered Ned Rector.

The pack mules had not yet come up with their driver. The party foreseeing this, had brought such articles as would be needed for the night. Taking their blankets and their rifles, together with food and wood for a fire, they began the slow, and what proved to be painful, ascent of Sunset Mountain.

A lava field stretched directly in front of them, barring the way. Its forbidding surface had been riven by the elements until it was a perfect chaos of black tumult. By the time the Pony Rider Boys had gotten over this rough stretch, they were ready to sit down and rest. Nance would not permit them to do so. He said they would have barely time to reach the crater before dark, as it was, and that they must make the best speed possible. No one grumbled except Stacy, but it was observed that he plodded along with the others, a few paces to the rear.

The Professor now and then would point to holes in the lava to show where explosions had taken place, bulging the lava around the edge and hurling huge rocks to a considerable distance. As they climbed the mountain proper they found that Sunset, too, had engaged in some gunnery in those far-away ages, as was shown by many lava bombs lying about the base.

The route up the mountain side was over a cider-buried lava flow, the fine cinders under foot soon making progress almost a torture. Tad was the first to stand on his head as his feet went out from under him.

Stacy, in a fit of uproarious laughter, did the next stunt, that of literally standing on his right ear. Chunky tried to shout and got his mouth full of cinders.

"I'm going back," howled the fat boy. "I didn't come up here to climb slumbering volcanoes."

"I'll tell you what I'll do, I'll carry you, Stacy," said Tad, smiling and nodding toward the cinder-blackened face of his companion.