Wading into one of the nation's most contentious environmental debates, U.S. Vice President Al Gore pledged today to prohibit logging and road building on 43 million acres of undeveloped national forests. Mr. Gore's promise goes beyond a proposal the Clinton administration made just three weeks ago that would bar road building across that same broad swath of roadless land in 155 national forests and grasslands. That land accounts for about a quarter of the entire forest system and includes many of its most pristine areas.