5. The best evidence for the layered-mantle thesis is the well-established fact that volcanic rocks found on oceanic islands， islands believed to result from mantle plumes arising from the lower mantle，are composed of material fundamentally different from that of the mid-ocean ridge system， whose source，most geologists contend，is the upper mantle. (5 -)

　　對(duì)于地幔分層論點(diǎn)來說，最好的證據(jù)乃這樣一個(gè)確認(rèn)的事實(shí)，即在那些海洋島嶼——這些島嶼據(jù)信是源于由下層地幔升上來的地幔柱狀溶巖流(mantle plume)——上發(fā)現(xiàn)的火山巖石，是由與海洋中部山脊系統(tǒng)的物質(zhì)根本不同的物質(zhì)構(gòu)成的，而這一海洋中部山脊系統(tǒng)的成因，大多數(shù)地質(zhì)學(xué)家論辯道，為上部地幔。

　　難句類型：復(fù)雜修飾、插入語

　　解釋：本句前面的主干并不算難，即the evidence is the fact that,難就難在fact之后說明fact 的同位語從句。此從句中既有大段插入語，又有從句，較為難讀。同位語從句的主語是volcanic rocks，主語后面的found on oceanic islands是分詞修飾volcanic rocks的;islands后面又有一個(gè)說明islands的同位語islands believed to result from mantle plumes arising from the lower mantle,此同位語實(shí)際起到一個(gè)分隔主謂的同位語的作用。其后是謂語動(dòng)詞 are composed/of fundamentally different from that of the midocean ridge system, whose source, most geologists contend, is the upper mantle。句中有一些專有詞匯。如果拋開文章不說，單看這一個(gè)句子，則句首的layered mantle thesis就難以理解。但是如果讀者能夠讀懂句子的大致意思，看到 layered mantle thesisr 最有力證據(jù)就是其材料 fundamentally different from… the upper mantle,這應(yīng)該可以推測(cè)出，既然不同于上層地幔，則這個(gè)理論一定是在說地幔有不同的層面。

　　意群訓(xùn)練：The best evidence for the layered-mantle thesis is the well-established fact that volcanic rocks found on oceanic islands，islands believed to result from mantle plumes arising from the lower mantle，are composed of material fundamentally different from that of the mid-ocean ridge system， whose source，most geologists contend，is the uppermantle.