Scientists have discovered still more evidence that an asteroid impact caused the demise of the dinosaurs 66 million years ago. The evidence consists of a layer of countless tiny glass spheres excavated below ground in the mountains of Haiti. The spheres, called tektites, result when molten rock, blasted into the air, forms droplets that cool into spheres. The layer lies between sediments deposited during dinosaur times and younger sediments above.

Researchers say the finding provides conclusive evidence of a major cosmic impact at the time dinosaurs disappeared. Although tektites can be formed by erupting volcanoes, the chemical composition of the Haitian glass points to a massive impact.

Timothy J. Bralower, geologist at the University of North Carolina at Chapel Hill who co-wrote a report on the tektites in the British science journal Nature, said this was the first time scientists could analyze well-preserved glass drops.

"There's no way that the composition had been derived from a volcanic eruption," Bralower said. "They {tektites} have way too much calcium."

Bralower said the calcium had to have come from the melting together upon impact of chalky marl sediments, which are common along the continental shelf. He and colleagues from the universities of Michigan, Florida and Rhode Island say the impact also released so much carbon dioxide from vaporized marl that it contributed to a "greenhouse effect."