杨鹏长难句里面的一个句子thisis,thedepletionofavailablenewland...ThehistorianFrederickJ.Turnerwroteinthe1890'sthattheagrariandiscontentthathadbeendevelopingsteadilyintheUnitedStatessinceabout1870hadbeenprec

Question

杨鹏长难句里面的一个句子 this is ,the depletion of available new land ...
The historian Frederick J.Turner wrote in the 1890's that theagrarian discontent that had been developing steadily in the UnitedStates since about 1870 had been precipitated by the closing of theinternal frontier——that is,the depletion of available new land　neededfor further expansion of the American farming system.
this is 到后面的部分是什么成分?修饰哪里?

Answer 1

the depletion of available new land　是 the closing of theinternal frontier的同位语,解释说明了边远地区封闭加剧的原因；而neededfor further expansion of the American farming system.则是
the depletion 的定语,意思是指对美国农业体系中进一步扩展的可用的新土地的消耗.