Certainly nothing of near-humanoid construction could ever have come into being on that planet without leaving some trace of themselves or their genetic forebears except for that single huge city.
How long the city had been there was anyone's guess. A thousand years? A million? There was no way of telling. It had been sealed tightly, so none of the sand that blew across the planet's surface could get in. It had been set on a high plateau of rock, far enough above the desert level to keep it from being buried, and the transparent dome was made of an aluminum oxide glass that was hard enough to resist the slight erosion of its surface that might have been caused by the gentle, thin winds dashing microscopic particles of sand against its smooth surface.
Inside, the dry air had preserved nearly every artifact, leaving them as they had been when the city was deserted by its inhabitants at an unknown time in the past.
That's right--deserted. There were no signs of any remains of living things. They'd all simply packed up and left, leaving everything behind.
Dating by the radiocarbon method was useless. Some of the carbon compounds in the various artifacts showed a faint trace of radiocarbon, others showed none. But since the method depends on a knowledge of the 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.