他们偶然发现一种稀有金属铱的含量在边界处突然变得很丰富，然后又慢慢地消失了。
They had found, accidentally, that a rare metal, iridium, suddenly became very abundant exactly at the boundary and then slowly fell away.
尤卡坦半岛上富含铱的沉积层和一个巨大的陨石坑提供了证据，证明了约6500万年前，一颗大型陨石撞击了地球。
An iridium-enriched sediment layer and a large impact crater in the Yucatn provide evidence that a large meteorite struck Earth about 65 million years ago.
然而，查特基博士的说法引发了疑问：为什么全球白垩纪晚期岩石中含有冲击石英和铱的喷发物地层只有一层？ 答案或许是，两次撞击是在同时发生的。
This story, though, raises the question of why there is but a single ejecta layer of iridium and shocked quartz in late Cretaceous rocks around the world.
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