But this thick layer of Iridium that separates the Cretaceous and Tertiary periods clearly was an anomaly and it had been deposited over a very short time.

但这个隔开了白垩纪和第三纪的厚铱层显然是个异常事物，而且它在很多的时间内便沉积下来。

Why this should be so we do not know; nor why the later tertiary periods should have been much more favourable for their existence than the present time.

数量之多无可比拟。 为什么会这样我们不知道； 也不知道为什么后来的第三纪时期应该比现在更有利于它们的存在。