red depth, converted it into electrical
energy and thence into an image which was flashed onto a screen.

At the depth of ten miles, the image was fuzzy, though good enough to
distinguish the main features of the strata. At three miles, it was
better. It could pick up the sound reflection of a buried coin and
convert it into a picture on which the date could be seen.

It was to a geologist as a microscope is to a biologist. Being a
biologist, Dano Marin could appreciate the analogy.

He started at the tip of the peninsula and zigzagged across, heading
toward the isthmus. Methodically, he covered the territory, sleeping
at night in the digger. On the morning of the third day, he discovered
oil traces, and by that afternoon he had located the main field.

He should probably have turned back at once, but now that he had found
oil, he investigated more deliberately. Starting at the top, he let
the image range downward below the top strata.

It was the reverse of what it should have been. In the top few feet,
there were plentiful fossil remains, mostly of the four species of
mammals. The squirrel-like creature and the far larger grazing animal
were the forest dwellers. Of the plains animals, there were only two,
in size fitting neatly between the extremes of the forest dwellers.

After the first few feet, which corresponded to approximately twenty
thousand years, he found virtually no fossils. Not until he reached a
depth which he could correlate to the Late Carboniferous age on Earth
did fossils reappear. Then they were of animals appropriate to the
epoch. At that depth and below, the history of Glade was quite similar
to Earth's.

Puzzled, he checked again in a dozen widely scattered localities. The
results were always the same--fossil history for the first twenty
thousand years, then none for roughly a hundred million. Beyond that,
it was easy to trace the thread of biological development.

In that period of approximately one hundred million years, something
unique had happened to Glade. What was it?

On the fifth day his investigations were interrupted by the sound of
the keyed-on radio.

"Marin."

"Yes?" He flipped on the sending switch.

"How soon can you get back?"

He looked at the photo-map. "Three hours. Two if I hurry."

"Make it two. Never mind the oil."

"I've found oil. But what's the matter?"

"You can see it better than I can describe it. We'll discuss it when
you get back."

*