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 -)

对于地幔分层论点来说，最好的证据乃这样一个确认的事实，即在那些海洋岛屿——这些岛屿据信是源于由下层地幔升上来的地幔柱状溶岩流(mantle plume)——上发现的火山岩石，是由与海洋中部山脊系统的物质根本不同的物质构成的，而这一海洋中部山脊系统的成因，大多数地质学家论辩道，为上部地幔。

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。

意群训练：

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.