4 weeks - still bubbling?

[Grimlohk]

So...
I recently made 10 gallons of a stout, and I put it into two carboys on January 16th.
After about 7 days, I secondaried both of them.
One of them appeared ready to bottle a week or so ago, but the other one is still bubbling. Theres only a couple square inches of surface area, but the bubbles are still half an inch thick or so.
One was pitched with white labs yeast, the other with re-hydrated beer yeast (friend supplied it).
I guess my question is:
Why would the beer still be fermenting? I can see bubbles coming up the side of the carboy still.

Im worried that it may have gotten contaminated and that the longer I let it sit there waiting for it to stop, the more they are going to sour my beer
Anyone got any advice?

 

[dpt222]

What does your hydrometer tell you? That's probably your best bet!

Which one is which though, white labs and friend supplied?

 

[malkore]

the bubbles...they mean nothing!

hydrometer is the ONLY way to know a beer is done. you may have racked earlier on both, and this one is just pissed off enough to show you.

 

[InkPouchMan]

Could it just be CO2 in solution? If indeed you racked one early, it coulda produced more in secondary, while the other woulda been CO2-free (almost) from the racking.