同学你好，该知识点来自沪江网校《新概念英语2、3、4册连读【双11专享班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
in the midst of 是一个介词词组，意思是“在……当中”
Angelo laid the gun carefully on the table, in the midst of brochures and other papers.
安杰洛小心翼翼地把枪放在桌上的一堆手册和文件中间。

which在这里引导定语从句，which指代the two hills

in the midst of which flowed two rivers  相当于
in the midst of the two hills flowed a great river

这句话涉及到两个知识点，一个是“介词+关系代词”，一个是倒装结构。

句子原本是 a great river flowed in the midst of which （the two hills）

因为关系代词which前有介词词组in the midst of，所以in the midst of which一起移动到了定语从句的句首，形成了“介词+关系代词”结构，也就是in the midst of which （the two hills）a great river flowed

in the midst of which （the two hills）a great river flowed
经过了倒装，变成了in the midst of which （the two hills）flowed a great river
谓语flowed放到了主语a great river之前。这是因为in the midst of which是介词短语+which结构，从句可以倒装。

祝同学学习进步~