that his charges would be killed, but there was nothing more that he
could do.

The bombarding continued, some explosions sounding near at hand, others
further down or up the Canyon, but each of sufficient force to send
shivers up and down the spines of the Pony Rider Boys. They never had
experienced anything approaching this.

"I'm going to stand up," declared Tad, rising to his feet. "I won't
be killed any quicker standing than lying down. Besides, I don't like
to shirk."

"Stand up if you want to, but keep close to the wall," ordered Dad,
himself rising to his feet.

One by one the boys got up, Professor Zepplin following the example of
the guide. They had to shout in speaking in order to make themselves
heard above the bombardment, the roaring of the river and the cataract
over their heads.

"What is going on up there?" shouted Tad.

"Mountain falling in!"

"I knew it! I knew it!" yelled Chunky. "I knew something would fall
down as soon as I got here."

No one laughed. The situation was too serious for laughter.

"Is it a land or a rock slide?" questioned Tad further.