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美国前寒武地层出露面积大约占其总面积的10％,但前寒武地层在显生代地层下面分布是很广泛的,特别在美国中部内地更为明显。前寒武地层出露区分布在苏必利尔湖区和阿迪龙达克山脉的加拿大地盾向南突出部分,以及中部内地往南的小内露层中。前寒武地层还分布在东部阿巴拉契亚山脉和西部科迪勒拉山带中由于受显生代形变作用而产生的较高隆起区内,但有关太平洋附近地区的前寒武地层是很少介绍过的。同位素年令测定资料表明,前寒武地层在时代上的变化范围很大,从35.5亿年到6
Pre-Cambrian stratigraphic outcrops account for about 10% of the total area in the United States, but the Precambrian strata are widely distributed below the Paleozoic strata, especially in the central United States. Precambrian formations are exposed in the south prominently of the Canadian Shield in the Lake Superior and Adirondack Mountains, and in the small inner dew of the Central Mainland to the south. Precambrian stratigraphy is also found in the higher uplift resulting from the generational deformations in the Appalachian Mountains in the east and Cordillera in the west, but the Precambrian formations in the vicinity of the Pacific Ocean are Rarely introduced. The isotopic dating data show that the Precambrian strata vary widely in the period from 3.55 billion years to 6