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)

史學家弗雷德里克.杰.特納(Frederick J.Turner)于十九世紀九十年代著述道，美國約自18世紀70年代以來一直在持續(xù)不斷發(fā)展的農民不滿，由于國內邊遠地區(qū)(internal frontier)的封閉而更趨加劇——亦即是說，美國農業(yè)系統(tǒng)進一步擴展所必需的可資利用的新土地幾近耗竭。

難句類型：

復雜修飾

解釋：

典型的句子套句子。主句是F.J.T. wrote that，賓語從句中主干是the agrarian discontent had been precipitated by the closing of the internal frontier,在從句的主語the agrarian discontent后面又來了一個定語從句。

意群訓練：

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.