Read the excerpt from "A Quilt of a Country." Once these disparate parts were held together by a common enemy, by the fault lines of world wars and the electrified fence of communism. With the end of the cold war there was the creeping concern that without a focus for hatred and distrust, a sense of national identity would evaporate, that the left side of the hyphen—African-American, Mexican-American, Irish-American—would overwhelm the right. What is the denotative meaning of fault lines in this excerpt?