ARLINGTON, Va. There are cracks in the white marble Tomb of the Unknowns at Arlington National Cemetery, and each year, those cracks go farther and deeper. The 74-year-old monument is not in danger of crumbling any time soon. But the cemetery is deciding whether to patch the fissures or replace the marble altogether. The tomb has been patched repeatedly, most recently in 1989. John Metzler, the cemetery's superintendent, says the cemetery could repair it, do nothing or buy an extra marble block and store it as a standby. He worries that some of the carved sculptures could eventually fall off....