第一个句子分块挺多：
1、主句是He described it as a very agreeable situation.
located within ...是过去分词作situation的后置定语
2、定语从句部分指...two small hills 【in the midst of which】 flowed a great river
介词短语提前,也就是a great river flowed in the midst of two small hills
3、第二个句子符合语法规则（不过我不知道是不是够合逻辑,分开说是I have a table.A bottle stands in the middle of it.--瓶子怎么会在“桌子”的中间呢?应该是在“桌面”的中间部位.楼主自己权衡吧,我只是说说感觉）