Moreover, all this applies only to units of time . Rocksare divided into quite separate units known assystems, series, and stages. A distinction is alsomade between late and early (referring to time) andupper and lower (referring to layers of rock). It canall get terribly confusing to nonspecialists, but to ageologist these can be matters of passion. "I haveseen grown men glow incandescent with rage overthis metaphorical millisecond in life's history," theBritish paleontologist Richard Fortey has writtenwith regard to a long-running twentieth-centurydispute over where the boundary lies between the Cambrian and Ordovician.

而且，这一切都只用做时间的单位。岩石的单位还另有一套，叫做系、段和期。而且，还有早、晚(指时间)之分和上、下(指岩层)之别。对于不是专家的人来说，这简直是一锅粥;但对于地质学家来说，这都可能是会动感情的东西。“我看到大人们为了生命史上一毫秒的问题争得脸红脖子粗。”英国的理查德·福蒂在谈到20世纪为寒武纪和奥陶纪的分界线而展开的旷日持久的辩论时这样写道。

At least today we can bring some sophisticated dating techniques to the table. For most of thenineteenth century geologists could draw on nothing more than the most hopeful guesswork.The frustrating position then was that although they could place the various rocks and fossils inorder by age, they had no idea how long any of those ages were. When Buckland speculated onthe antiquity of an Ichthyosaurus skeleton he could do no better than suggest that it had livedsomewhere between "ten thousand, or more than ten thousand times ten thousand" yearsearlier.

今天，我们至少可以使用某些先进的技术来确定年代。在19世纪的大部分时间里，地质学家们只能依赖于推测。他们可以按照时代来排列各种岩石和化石，但根本不知道这些年代的长短，这是很令人泄气的。当巴克兰推测一副鱼龙骨骼的古老程度的时候，他只能认为，它生活在大约“10000或10000以上乘以10000”年以前。