The historian Frederick J. Turner wrote in the 1890's that the agrarian discontent that had been developing　steadily in the United States since about 1870 had been precipitated by the　closing of the internal frontier——that is,the depletion of available new land　needed for further expansion of the American farming system. (4)

史学家弗雷德里克.杰.特纳(Frederick J.Turner)于十九世纪九十年代著述道，美国约自18世纪70年代以来一直在持续不断发展的农民不满，由于国内边远地区(internal frontier)的封闭而更趋加剧——亦即是说，美国农业系统进一步扩展所必需的可资利用的新土地几近耗竭。

难句类型：

复杂修饰

解释：

典型的句子套句子。主句是F.J.T. wrote that，宾语从句中主干是the agrarian discontent had been precipitated by the closing of the internal frontier,在从句的主语the agrarian discontent后面又来了一个定语从句。

意群训练：

The historian Frederick J. Turner wrote in the 1890's that the agrarian discontent that had been developing　steadily in the United States since about 1870 had been precipitated by the　closing of the internal frontier——that is,the depletion of available new land　needed for further expansion of the American farming system.