another development—wholly unrelated on the face of it, 另一件大事——表面看来完全没有关系，

　　was quietly unfolding in Italy with the work of a young geologist from the Lamont Doherty Laboratory at Columbia University. 在意大利悄悄揭开序幕，由于哥伦比亚大学莱蒙特·多赫蒂实验室的一位年轻地质学家的工作。

　　In the early 1970s, Walter Alvarez was doing fieldwork in a comely defile known as the Bottaccione Gorge, near the Umbrian hill town of Gubbio, 20世纪70年代初，在距离翁布里亚地区的山城古比奥不远的地方，沃尔特·阿尔瓦雷斯正在一个名叫博塔西昂峡谷的峡道里作实地考察。

　　when he grew curious about a thin band of reddish clay that divided two ancient layers of limestone, 他突然对薄薄的一层淡红色黏土发生了兴趣。

　　one from the Cretaceous period, the other from the Tertiary. 这层黏土把古代灰岩分为两层，一层属于白垩纪，一层属于第三纪。

　　This is a point known to geology as the KT boundary, and it marks the time, sixty-five million years ago, 这在地质学里被称之为KT界线。它标志着6500万年前

　　when the dinosaurs and roughly half the world's other species of animals abruptly vanish from the fossil record. 恐龙和世界上大约一半其他种类的动物从化石记录中突然消失。

　　It is KT rather than CT because C had already been appropriated for Cambrian. 是KT界线，而不是CT界线，因为C已经用来代表“寒武纪”。

　　Depending on which source you credit, the K comes either from the Greek Kreta or German Kreide. K不是来自希腊语Kreta，就是来自德语Kreide。

　　Both conveniently mean "chalk," which is also what Cretaceous means. 二者的意思都是白垩，这也是“白垩纪”的意思。

　　Alvarez wondered what it was about a thin lamina of clay, barely a quarter of an inch thick, 阿尔瓦雷斯不大明白，只有6毫米左右厚的薄薄一层黏土，

　　that could account for such a dramatic moment in Earth's history. 怎么能说明地球史上这么个戏剧性的时刻。

　　At the time the conventional wisdom about the dinosaur extinction was the same as it had been in Charles Lyell's day a century earlier, 当时，关于恐龙灭绝的时间，人们通常的看法与一个世纪前查尔斯·莱尔时代一样，

　　namely that the dinosaurs had died out over millions of years. 即恐龙灭绝于几百万年以前。