amount of nitrogen in the atmosphere of the planet of origin, the rate
of bombardment of that atmosphere by high-velocity particles, and
several other factors, the information on the radioactivity of the
specimens meant nothing. There was also the likelihood that the carbon
in the various polymer resins came from oil or coal, and fossil carbon
is useless for radio-dating.

Nor did any of the more modern methods show any greater success.

It had taken Man centuries of careful comparison and cross-checking to
read the evolutionary history written in the depths of his own planet's
crust--to try to date the city was impossible. It was like trying to
guess the time by looking at a faceless clock with no hands.

There the city stood--a hundred miles across, ten thousand square miles
of complex enigma.

It had given Man his first step into the ever-widening field of Cultural
Xenology.

Dave Turnbull finished his sherry, got up from the breakfast nook, and
walked into the living room, where his reference books were shelved. The
copy of Kleistmeistenoppolous' "City of Centaurus" hadn't been opened in
years, but he took it down and flipped it open to within three pages of
the section he was looking for.

"It is obvious, therefore, that every one of the indicators
points in the same direction. The City was not--_could not have
been_--self-supporting. There is no source of organic material on the
planet great enough to support such a city; therefore, foodstuffs must
have been imported. On the other hand, it is necessary to postulate
_some_ reason for establishing a city on an otherwise barren planet and
populating it with an estimated six hundred thousand individuals.

"There can be only one answer: The race that built the City did so for
the same reason that human beings built such megalopolises as New York,
Los Angeles, Tokyo, and London--because it was a focal point for
important trade routes. Only such trade routes could support such a
city; only such trade routes give reason for the City's very existence.

"And when those trade routes changed or were supplanted by others in the
course of time, the reason for the City's existence vanished."

Turnbull closed the book and shoved it back into place. Certainly the
theory made sense, and had for a century. Had Duckworth come across
information that would seem to smash that theory?

The planet itself seemed to be perfectly constructed for a gigantic
landing field for inte