主语：he
谓语：described
宾语：it
宾补： a very agreeable situation located within two hills in the midst of which flowed a greet river. 其中 as 起引导作用,与think sb as ...类似.
至于 of which 后的flowed a greet river,其所在句子“in the midst of which flowed a greet river.”就是一个倒装句,由表示位置的in短语置于句首引导.具体地所,该倒装句主语是 a greet river.懂了吧?!