SAN FRANCISCO (AP).- The big dig for San Francisco’s multibillion dollar transportation terminal has unearthed some artifacts from the city’s heady Gold Rush days, including opium pipes from a Chinese laundry and a chipped chamber pot found in a backyard outhouse. The 70 artifacts have city archaeologists eager for more and local residents pondering the ground beneath their feet. “It’s not often that you get a chance to stop for a moment and have a window into what used to be,” said James M. Allan, an archaeologist with William Self Associates, the firm ensuring the items are unearthed and preserved. “It gives you pause.” The $4 billion Transbay Transit Center under construction in the South of Market financial district is billed as the “Grand Central Station of the West.” The 1 million-square-foot bus and train station will serve as the northern end of California’s planned high-speed rail between