1. The historian Frederick J. Turner wrote in the 1890's that the agrariandiscontent that had been developing steadily in the United States since about1870 had been precipitated by the closing of the internal frontier——that is, thedepletion of available new land needed for further expansion of the Americanfarming system.
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 beenprecipitated by the closing of the internal frontier,在从句的主语the agrariandiscontent后面又来了一个定语从句。