复句有多重复句，至少有两重，其层次可由多变少，原语多层复句关系压缩为相对少的复句关系。如：

On many steep mountain slopes the erosional agents are so active that they even scour and remove fresh rock before it has been appreciably softened by weathering.

在许多陡峭的山坡上，侵蚀营力十分活跃，甚至于在风化作用使新鲜岩石明显地变松软以前就把它冲刷和搬运走了。

原文为三层复句关系，译文只有两层复句关系。

3. 句群内合

1) 句群内合为单句

—个意思可以分几句说，也可以合说，作者与译者都有很大的灵活性。如：

Such fissure fillings, called fissure veins, are one of the most important kinds of deposit. They are the chief source of gold, silver, copper. (Many such veins have been followed down thousands of feet and ultimately have had to be abandoned because of working difficulties.)

句群：被称之为裂缝脉的这种裂缝充填物，是最重要的矿床类型之一。它们是金、银、铜的主要来源。 (……）

单句：被称之为裂缝脉的这种裂缝充填物，是金、银、铜的主要来源和最重要的矿床类型之一。

2)句群内合为复句

如前所述，复句与句群的划界并不十分严格，全在于作者和译者的主观决定。所以句群还可以内合为复句。如：

To begin with, we can divide ore deposits into two large classes. There are those which are primary and are now found in the positions in which they were originally formed, and there are secondary ores which have been transported from their original position by some agency.

首先，我们可以把矿床分为两大类：一类是原生矿床， 现今仍在原来产生的位置；还有一类是次生矿床，是被某种 地质营力从原生地点搬运来的。

原句是个句群，由两个句子构成，译者将两句间的句号換为冒号，译句就变成了复句。