难句类型：复杂修饰、插入语
解释：本句前面的主干并不算难，即the evidence is the fact that,难就难在fact之后说明fact 的同位语从句。此从句中既有大段插入语，又有从句，较为难读。同位语从句的主语是volcanic rocks，主语后面的found on oceanic islands是分词修饰volcanic rocks的；islands后面又有一个说明islands的同位语islands believed to result from mantle plumes arising from the lower mantle,此同位语实际起到一个分隔主谓的同位语的作用。其后是谓语动词are composed/of fundamentally different from that of the midocean ridge system, whose source, most geologists contend, is the upper mantle。
句中有一些专有词汇。如果抛开文章不说，单看这一个句子，则句首的layered mantle thesis就难以理解。但是如果读者能够读懂句子的大致意思，看到layered mantle thesisr最有力证据就是其材料fundamentally different from… the upper mantle,这应该可以推测出，既然不同于上层地幔，则这个理论一定是在说地幔有不同的层面。
 

2. some geologists，however，on the basis of observations concerning mantlexenoliths， argue that the mantle isnot layered，but that heterogeneity is created by fluids rich in "incompatible elements" (elements tending towardliquid rather than solid state)percolating upward and transformingportions of the upper mantleirregularly，according to the vagaries of the fluids’ pathways. (4+)
但是，某些地质学家，以对地幔捕虏岩体(xenolith)所作的观察为依据，指出地幔并非是分层排列的，相反，地幔的异质性是由那些富含“不相容成分”的流质构成的，这些成分趋向于流体而非固体的状态，自下而上渗透扩散，并依照这些流质流向的任意性，不规则地将上部地幔的某些部分予以改变。
 

难句类型：复杂修饰、插入语
解释：本句的主句中有一个长插入语however,on the basis of observations concerning mantle xenoliths,割裂了主谓，但句子的主要难度还是在that引导的宾语从句中，从句中有由but 所连接的两个句子，一起延续到句末。首选是rich in incompatible elements修饰fluids；后面又有一个同位语和一个分词结构，都是修饰incompatible elements的，最后还有一个状语来修饰那个分词结构，层层修饰，比较复杂。

　　
三、文章结构
Ts是文章的第一句，因此这一篇短文章是结论解释型。文章开始说出了地幔是heterogeneous,然后又指出了他的spatial arrangement remains unresolved,然后提出了两种理论，一是layyered-mantle thesis,另外一个是irregularly heterogeneous.然后分别对这两种理论进行describe.最后作者指出，We believe,perhaps unimaginatively, that this debate can be resolved through further study, and that the underexplored midocean ridge system is the key.