be
safe, for the young men were led by one of the regular mountain guides;
and besides, there was a track in the sand, showing that other parties
had gone down before. So he said that Rollo and Josie might go.

"You may go down with this party," said Mr. George, "and then you can
come up and take care of Rosie while I go down with _our_ guide."

So Rollo and Josie followed the two young men down. Mr. George watched
them from above. They went down very easily, for the sand was soft, and
the track turned this way and that, so as to avoid the steepest places.
The black lava covered the whole floor of the crater, and Mr. George and
Rosie supposed that those who had gone down would be able only to go to
the edge of it; but, to their great surprise, they found that the guide,
as soon as he reached it, stepped upon it, and walked boldly out,
followed by the young men and by Rollo and Josie, like a party of boys
walking out upon the ice on a pond.

"Why, uncle George!" exclaimed Rosie, "they are walking over the lava.
Why don't they sink in?"

"I cannot imagine," said Mr. George. "I supposed it was soft."

In fact, it _was_ soft; that is, it was soft enough to flow if it had
been on a slope, but yet it was hard enough to walk upon. A current of
lava, when it is coming down the mountain side, can often be walked upon
while it is still in motion. Its fluidity at the best is very imperfect,
and its motion is very slow. The lava which Rollo was upon in the floor
of the crater, though pretty nearly cool and hard on the surface, was
hot below. Rollo could see the redness of the heat in the holes and
crevices. Probably, if a heavy stone were laid upon the bed of lava, it
would gradually have sunk into it. And yet persons could walk over it
without any difficulty.

Rollo and Josie followed the young men over the lava until they came so
near the cone in the centre that if they were to advance farther they
would be in danger of having the lava which was thrown up from it fall
upon their heads. Here they found some boys, who belonged to the
mountain, engaged in getting out small pieces of the lava, where it was
hot and soft, and pressing coins into it, to sell to the people above.
Rollo and Josie bought some of these specimens of the boys, and put them
hot in their pockets.

While the boys were thus near the cone in the centre of the crater,
they were sometimes lost to view from Mr. George and Rosie, on account
of the puffs of vap