Home prices in major U.S. cities rose at the slowest rate in two years, according to the S&P/Case Shiller composite index of 20 metropolitan areas released Tuesday. The index gained 6.7% in the year ending in July. That was just shy of expectations. While new home purchases surged 18% in August to the highest level in more than six years, sales of existing homes unexpectedly fell last month from a year earlier. Construction and housing starts are down as well.