y a casual glance.

In a few minutes more they had left it behind and had drawn close to
its bigger sister. Choosing a place at which the timber seemed
thinnest they ran the canoe up on shore and fastened it securely.

With guns in hand they scrambled up the high bank and stood for a
moment surveying the surroundings. From that elevation, they could see
quite clearly for a couple of miles in each direction. Save for the
little island they had passed they could see no other solid land within
the range of their vision.

Charley noted the fact with satisfaction. "The solution of our mystery
must lie on one of these two islands," he declared, "and the chances
are in favor of this one, so here goes to discover it," and he plunged
into the timber with Walter close at his heels. He had taken no more
than twenty steps when he stopped with an exclamation of surprise and
astonishment, his way was barred by a great wall of stone that towered
several feet above his head. It had once been a fortification of
considerable strength, but growing trees had made breaches in it here
and there, their thrusting, up-growing trunks tumbling its blocks to
the ground, where they lay hidden by covering vines.

"Whew," whistled Walter as he readied his chumps side, "who could have
built this? It could hardly have been done by the Seminoles."

"No," said Charley, who was examining the strange wall carefully, "this
stone is all limestone, which is found only along the coast or at a
great depth. It has been brought here from a considerable distance.
Indians may have done the work, but they never did it willingly. If
they did it at all, it was as slaves. But we have no time for idle
speculation. Let's walk along it and see how far it extends."

But after forcing their way along the wall for almost a quarter of a
mile, at the expense of a good deal of exertion, they gave up the task.

"I believe it extends clear around the island," Walter declared, "we
can't spare any more time to follow it up; it's noon already. Let's
see what is inside."

Charley offered no objection, and the two boys climbed through a gap in
the wall and reached the great enclosure.

At first glance, they could see but little difference between the dense
growth amongst which they stood and that outside the wall, but a closer
examination showed that, while the timber was very thick, it was of
smaller size than that which they had left behind.

"This was a clear