Image source: Harley-Davidson.Friday finally brought some semblance of calm to Wall Street after a week that saw extensive volatility in the aftermath of the U.K. Brexit vote. Yet markets refused to give up any ground, and the S&P 500 climbed back over the 2,100 mark as major stock indexes posted modest gains of 0.1% to 0.4% on the day.Bond yields fell to record low levels as market participants question whether sluggish global economic trends will ever put central banks in a position to boost interest rates again.