TPO听力中的一段话 结论怎么理解呢

mr_colorUID: 3412299

下面这段话是TOp1的第2个lecture中的一部分。后面为甚么会说用Uranium-lead dating这种方法可以肯定关于大峡谷的假设是错的呢，这个assumpation是什么没看懂呢？ 求指教！
Uranium-Lead Dating allowed them to go about it in an entirely different way. What they did was: they looked at the grains of Zircon in the sandstone. Zircon is a material that contains radioactive Uranium, which makes it very useful for dating purposes. Zircon starts off as molten magma, the hot larva from volcanoes. This magma then crystallizes. And when Zircon crystallizes, the Uranium inside it begins to change into Lead. So if you measure the amount of Lead in the Zircon grain, you can figure out when the grain was formed. After that, you can determine the age of Zircon from different mountain ranges. Once you do that, you can compare the age of the Zircon in the sandstone in your sample to the age of the Zircon in the mountains. If the age of the Zircon matches the age of one of the mountain ranges, then it means the sandstone actually used to be part of that particular mountain range. Is everybody with me on that? Good.So, in this case, Uranium-Lead Dating was used to establish that half of the sandstone in the samples was formed at the same time the granite in the Appalachian Mountains was formed. So because of this, this new way of doing Uranium-Lead Dating, we’ve been able to determine that one of our major assumptions about the Grand Canyon was wrong.

ZTreeUID: 3165682

这一段前面3/4都是在讲怎么利用zircon测年代
因为zircon里的uranium会随着时间衰变成lead，所以只要测出zircon中lead的含量就可以测定出这个sample的年代了
倒数第四行lecturer说，通过这种方法人们发现这些sample中有一半是和appalachian mt同一时代的，所以之前的一个assumption是错的
至于assumption是啥，你问我我问谁，回去看上下文有没有和这个结论矛盾的假设就是了