It was Sikaso!

The next minute would have been Diego's last but that his two
followers lifted him to his feet and, picking him up like a child,
ran for his canoe with him. With a few rapid strokes they were in
midstream and paddling up the river with powerful strokes while
Sikaso raged impotently on the shore.

"Oh for one of the white men's fire-tubes!" he sighed, and even as
he spoke a sharp reminder of the efficiency of these same
"fire-tubes" whizzed past his ear in the shape of a bullet from
Diego's revolver.

In a few steps the old black was beside his young leader and with a
couple of strokes of his keen blade had set him free.

"Quick, Sikaso; the canoes--we must pursue him. Call the boys and
Ben while I cast off the canoes. Quick, we have not a minute to
lose."

Although Diego in his hurry had not carried off the map but left it
lying on the ground, still Frank realized that the Portuguese had
not actually needed the document to aid Muley-Hassan to find the
cache. The Arab was no doubt familiar with the location anyway, but
to head off all danger of the boys getting there first, it was vital
to stop Diego at all costs. In a few bounds Frank reached the
little indentation in the bank where the canoes were kept.

As he gained it he fell back with a groan and, brave boy as he was,