By correlating markings made in various parts of the world, historians have been able to read this difficult code. They have found that it is connected with the passage of days and the phases of the moon. It is, in fact, a primitive type of calendar.

　　13.By correlating markings made in various parts of the world, historians have been able to read this difficult code.

　　历史学家通过把世界各地留下的这种痕迹放在一起研究，终于弄懂了这种费解的代码。

　　语言点1:句子结构分析:by引导方式状语，指“通过…方式”，其中的made in various parts of the world是过去分词短语，作markings的后置定语。

　　语言点2:have been able to read不可理解为“已经能够读了”，read在此引申为“弄懂”。

　　14.They have found that it is connected with the passage of days and the phases of the moon.

　　他们发现代码与昼夜更迭和月亮圆缺有关。

　　语言点1:句子结构分析:that引导宾语从句，交代have found的内容;这个宾语从句中的and连接两个并列的宾语。

　　语言点2:sth.be connected with sth.的意思是“某物与某物之间存在联系”。

　　15.It is, in fact, a primitive type of calendar.

　　事实上是一种最原始的日历。

　　语言点:句子结构分析:in fact不仅是插入语，还起强调作用。It指上文中所说的code(代码)。