A day after the S&P 500's first close above 2,000, veteran trader Art Cashin told CNBC on Wednesday the rally could soon lose steam, at least if history is any indicator. "Unfortunately, history gives us a kind of muted picture of it, that if you have high volume, you get follow-through very easily. If you have low volume and you go into September, we've only had big carry-through a few times," Cashin, director of floor operations at the NYSE for UBS, said on "Squawk on the Street." Much of the recent gains have come on low volume, suggesting investors are either wary of jumping in at current levels, or have simply ... (full story)