Just a day after an almost incredible recovery for the S&P 500 from a 4% rout, the U.S. benchmark gauge resumed its losses. While the gauge traded off its worst point of the day, it still headed toward its lowest level since October. The Nasdaq 100 tumbled more than 3%. Amid violent equity swings, the Cboe Volatility Index extended its surge into a sixth consecutive day, and was poised for a one-year high. The dollar rose, while 10-year Treasuries were little changed.