Paul's first lesson in geology, and it gave him his first idea
that this hard slate, and the veins of coal enclosed between its solid
walls, might have had a previous existence in another form. He pondered
upon the length of time that must have passed since those ferns grew,
and since that running bird made those footprints, and finally concluded
to ask Derrick if he knew.

At noon, after Harry Mule had been sent jingling to his stable, Derrick
rejoined his friend, and they ate lunch together. As they talked of the
strange markings on the walls, and Derrick confessed that he knew no
more concerning their age than Paul, the latter suddenly paused, and
with a slight gesture directed attention to something in the roadway.

Looking in the direction indicated, Derrick saw, sitting bolt-upright on
its hind-legs, and gazing steadily at them, an immense rat. He was quite
gray, and evidently very old; nor did he seem to be in the least bit
afraid of them.

"Doesn't he look wise?" whispered Paul.

"As wise as Socrates," answered Derrick.

Not having had Derrick's education, Paul did not know who Socrates was,
but the name pleased him, and he said it over softly to
himself--"Socrates, Soc, Socrates. That's what I'm going to call him,
Derrick--'Socrates.' I've seen him round here two or three times this
morning, and every time he's sat up just like that, and looked as if he
knew all that I was thinking about. I believe he could tell how old the
ferns are."

"I don't believe they're as old as he is," replied Derrick, laughing.

The rat did not seem to like this, for at Derrick's laughter he gave a
little squeak and darted away, disappearing beneath the door.

Within five minutes Paul pointed again, and there sat the rat in
precisely the same position as before.

"Perhaps this is what he wants," said Paul, throwing a bit of bread
towards the rat. Approaching it cautiously, the beast first smelled of
it, and then seizing it in his mouth again darted beneath the door.
Several times did he thus come for food, but he always carried it away
without stopping to eat even a crumb.

"He must have a large and hungry family," said Derrick.

"Or else it isn't his dinner-hour yet, and he is waiting for the proper
time to eat," laughed Paul.

Always after this Socrates the rat was a regular attendant upon the boys
at lunch-time, and he never failed to receive a share of whatever they
had to eat. Often at other times, when no sound s