ing in its orbit, the end of the tail--in
order to keep its position--must move with a rapidity impossible to
conceive. If this tail were composed of nebulous mist, or anything of
that sort, it could not keep its position. There is only one theory
which could account for this position, and that is that the head of
a comet is a lens and the tail is light. The light of the sun passes
through the lens and streams out into space, forming the tail, which
does not follow the comet in the inconceivable manner generally
supposed, but is constantly renewed, always, of course; stretching away
from the sun!"

"Oh, dear!" ejaculated Margaret. "I have read that."

"A little patience," he said. "When I arrived at the bottom of the
shaft, I found myself in a cleft, I know not how large, made in a vast
mass of transparent substance, hard as the hardest rock and transparent
as air in the light of my electric lamps. My shell rested securely upon
this substance. I walked upon it. It seemed as if I could see miles
below me. In my opinion, Margaret, that substance was once the head of a
comet."

"What is the substance?" she asked, hastily.

"It is a mass of solid diamond!"

Margaret screamed. She could not say one word.

"Yes," said he, "I believe the whole central portion of the earth is
one great diamond. When it was moving about in its orbit as a comet, the
light of the sun streamed through this diamond and spread an enormous
tail out into space; after a time this nucleus began to burn."

"Burn!" exclaimed Margaret.

"Yes, the diamond is almost pure carbon; why should it not burn? It
burned and burned and burned. Ashes formed upon it and encircled it;
still it burned, and when it was entirely covered with its ashes it
ceased to be transparent, it ceased to be a comet; it became a planet,
and revolved in a different orbit. Still it burned within its covering
of ashes, and these gradually changed to rock, to metal, to everything
that forms the crust of the earth."

She gazed upon him, entranced.

"Some parts of this great central mass of carbon burn more fiercely
than other parts. Some parts do not burn at all. In volcanic regions the
fires rage; where my great shell went down it does not burn at all. Now
you have my theory. It is crude and rough, for I have tried to give it
to you in as few words as possible."

"Oh, Roland," she cried, "it is absurd! Diamond! Why, people will think
you are crazy. You must not say such a thin