Art Cashin said he's carefully watching the Nasdaq Composite, the stock market's current "canary in the coal mine." The bulls are happy it successfully defended support around 4015, but remain cautious.
Cashin, UBS' director of floor operations at the NYSE, also told CNBC's Bob Pisani around midday that yesterday's weak auction of U.S. 30-year bonds put the stock market on edge. "The 30-year auction looked like it was going to be OK. It turned into be a real disaster. That had people thinking, 'Wait a minute. Money may be running out. Maybe it's not as readily available as we thought.' That had them turn on the stock market, in particular the momentum stocks (in the Nasdaq.) So that's still there."
—By CNBC's Alex Crippen. Follow him on Twitter