ed by Thurid and
the therns he emitted a single low growl, and when he came beside
me upon the ledge and my hand rested upon his neck I felt his short
mane bristling with anger. I think he sensed telepathically the
recent presence of an enemy, for I had made no effort to impart to
him the nature of our quest or the status of those we tracked.

This omission I now made haste to correct, and, after the manner
of green Martians with their beasts, I let him know partially by
the weird and uncanny telepathy of Barsoom and partly by word of
mouth that we were upon the trail of those who had recently occupied
the boat through which we had just passed.

A soft purr, like that of a great cat, indicated that Woola
understood, and then, with a word to him to follow, I turned to
the right along the ledge, but scarcely had I done so than I felt
his mighty fangs tugging at my leathern harness.

As I turned to discover the cause of his act he continued to pull
me steadily in the opposite direction, nor would he desist until
I had turned about and indicated that I would follow him voluntarily.

Never had I known him to be in error in a matter of tracking, so
it was with a feeling of entire security that I moved cautiously in
the huge beast's wake. Through Cimmerian darkness he moved along
the narrow ledge beside the boiling rapids.

As we advanced, the way led from beneath the overhanging cliffs
out into a dim light, and then it was that I saw that the trail
had been cut from the living rock, and that it ran up along the
river's side beyond the rapids.

For hours we followed the dark and gloomy river farther and farther
into the bowels of Mars. From the direction and distance I knew
that we must be well beneath the Valley Dor, and possibly beneath
the Sea of Omean as well--it could not be much farther now to the
Temple of the Sun.

Even as my mind framed the thought, Woola halted suddenly before a
narrow, arched doorway in the cliff by the trail's side. Quickly
he crouched back away from the entrance, at the same time turning
his eyes toward me.

Words could not have more plainly told me that danger of some sort
lay near by, and so I pressed quietly forward to his side, and
passing him looked into the aperture at our right.

Before me was a fair-sized chamber that, from its appointments, I
knew must have at one time been a guardroom. There were racks for
weapons, and slightly raised platforms for the sleeping silks and