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 fagged. The animals were turned out for the night after
having been hobbled so that they could not stray far away.