Scientists felt that they could get an idea of how long the extinctions took by determining how long it took to deposit this one centimeter of clay and they thought they could determine the time it took to deposit the clay by determining the amount of the element iridium (Ir) it contained. (TPO8, 52)美国大学托福

extinction /ɪk'stɪŋkʃən/ n. 灭绝，绝种;熄灭

deposit /dɪ'pɑzɪt/ v. 将(钱)存入银行; 付(定金);放下某物;使(淤泥等)沉积

clay /klei/ n. 粘土

我是分界线，大家先自己分析哦。

Scientists felt that they could get an idea (of how long the extinctions took) (by determining)(how long it took to deposit this one centimeter of clay) and they thought they could determine the time (it took to deposit the clay)(by determining the amount of the element iridium (Ir) ) (it contained. ) (TPO8, 52)

分析：

修饰一：(of how long the extinctions took)，介词短语

中文：灭绝所用的时间

修饰二：(by determining) ，介词短语

中文：通过确定

修饰三：(how long it took to deposit this one centimeter of clay) ，从句

中文：这层一厘米黏土层的沉积时间

修饰四：(it took to deposit the clay) ，从句，修饰time

中文：这层黏土层的沉积时间

修饰五：(by determining the amount of the element iridium (Ir) ) ，介词短语

中文：通过确定黏土层包含铱的含量

修饰六：(it contained. ) ，从句，修饰iridium

中文：它(黏土层)所包含的

参考翻译：

科学家感觉他们可以通过确定这层一厘米黏土层的沉积时间就可以知道灭绝所用的时间，而他们认为可以通过确定黏土层包含铱的含量能确定它(黏土层)所沉积的时间。(有点绕)

这个句子的主要修饰成分就是从句和介词短语，大家务必想清楚，每一个修饰成分修饰的是什么，才能看清楚这个句子的结构。