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　　Geologists have long known that the Earth‘s mantle is heterogeneous， but its spatial arrangement remains unresolved―is the mantle essentially layered or irregularly heterogeneous? 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 midocean ridge system， whose source， most geologists contend， is the upper mantle.Some geologists， however， on the basis of observations concerning mantle xenoliths， argue that the mantle is not layered， but that heterogeneity is created by fluids rich in "incompatible elements" (elements tending toward liquid rather than solid state) percolating upward and transforming portions of the upper mantle irregularly， according to the vagaries of the fluids‘ pathways. We believe， perhaps unimaginatively， that this debate can be resolved through further study， and that the underexplored midocean ridge system is the key.

　　一、難詞

　　spatial： a.1.空間的，太空的;2.存在(或者發(fā)生)于宇宙空間的

　　plume： n.1.羽狀物2.柱，地柱

　　xenolith：俘虜巖

　　percolate：v.1.滲透2.擴散到，彌漫于

　　portions： n.一部分

　　二、難句

　　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)學家論辯道，為上部地幔。

　　難句類型：復雜修飾、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。

　　some geologists，however，on the basis of observations concerning mantle　xenoliths， argue that the mantle is　not layered，but that heterogeneity is created by fluids rich in "incompatible elements" (elements tending toward　liquid rather than solid state)percolating upward and transforming　portions of the upper mantle　irregularly，according to the vagaries of the fluids‘ pathways. (4+)

　　但是，某些地質(zhì)學家，以對地幔捕虜巖體(xenolith)所作的觀察為依據(jù)，指出地幔并非是分層排列的，相反，地幔的異質(zhì)性是由那些富含“不相容成分”的流質(zhì)構成的，這些成分趨向于流體而非固體的狀態(tài)，自下而上滲透擴散，并依照這些流質(zhì)流向的任意性，不規(guī)則地將上部地幔的某些部分予以改變。

　　難句類型：復雜修飾、插入語

　　解釋：本句的主句中有一個長插入語however，on the basis of observations concerning mantle xenoliths，割裂了主謂，但句子的主要難度還是在that引導的賓語從句中，從句中有由but 所連接的兩個句子，一起延續(xù)到句末。首選是rich in incompatible elements修飾fluids;后面又有一個同位語和一個分詞結(jié)構，都是修飾incompatible elements的，最后還有一個狀語來修飾那個分詞結(jié)構，層層修飾，比較復雜。

　　三、文章結(jié)構

　　Ts是文章的第一句，因此這一篇短文章是結(jié)論解釋型。文章開始說出了地幔是heterogeneous，然后又指出了他的spatial arrangement remains unresolved，然后提出了兩種理論，一是layyered-mantle thesis，另外一個是irregularly heterogeneous.然后分別對這兩種理論進行describe.最后指出，We believe，perhaps unimaginatively， that this debate can be resolved through further study， and that the underexplored midocean ridge system is the key.