Enclosed Seas Important Earth's Require Explanation Addition Island

意群训练： The enclosed sea s are an important featur e of the earth's surface and seriousl y require explanation because ，in addition t o the enclosed seas that are developin g at presen t behind island arcs ，there are a number of older ones o f possibly similar origin ，such a s the Gulf of Mexico , the Black Sea ，and perhap s the North Sea .

标志： The enclosed seas are an important feature (of the earth's surface) and seriously require explanation /because ，in addition to the enclosed seas (that are developing at present behind island arcs), there are a number of older ones of possibly similar origin，such as the Gulf of Mexico, the Black Sea, and perhaps the North Sea/.

译文： 内海 是 （地球表面的）一个重要特征 ，而且 急需 解释 ，/因为 除了当前在弧状岛屿链之后发展的内海之外， 还有一些更老的内海可能有相似的成因，比如墨西哥湾、黑海、甚至有可能北海都是如此。/

解释： 主句是一个由and相连接的并列句。because之后的所有内容都是主句的状语从句。然而because和在意思上与其连贯的there are...被比较长的插入语分割，容易使读者的思路不连续。不过本句在意思上不难，读者读起来不至于感到太困难。