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 -)

對于地幔分層論點來說，最好的證據(jù)乃這樣一個確認的事實，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后面又有一個說明islands的同位語islands believed to result from mantle plumes arising from the lower mantle,此同位語實際起到一個分隔主謂的同位語的作用。其后是謂語動詞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)該可以推測出，既然不同于上層地幔，則這個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，islandsbelieved to result from mantle plumes arising from the lower mantle，arecomposed of material fundamentally different from that of the mid-ocean ridge system， whose source，most　geologists contend，is the upper mantle.