He shouldn't have been seperated, per se, from the sheep by something, as you won't know if he could herd or not by doing that. Herding is a dog moving livestock. Not looking at livestock, but physically moving them with there body/presence by circles, half circles, driving, fetching, etc. Like you seem to have learned chasing, like Jax was doing, isn't herding. I think if it had been in a more controled situation, you'd have probably been suprised what Jax would do - since he seems to have alot of natural talent.