"Flow, not flood," corrected the professional automatically. "We're not in sight of it yet. We've just rounded the corner that takes us out of your sight. It's over a mile yet, and a couple of more corners, before we get to the spot where I left it. Of course, it will be closer than that by now. It was spreading at perhaps a hundred yards an hour then. That's one figure we must refine.... Of course, I'll try to get samples, too. I wish there were some way to get samples of the central cone. The whole thing is the queerest volcano I've ever heard of. Have you gotten Eileen started back?" "Not as far as I can tell. As with your cone samples, there are practical difficulties," replied Zaino. "I haven't quit yet, though." "I should think not. If some of us were paid by the idea we'd be pretty poor, but the perspiration part of genius is open to all of us." "You mean I should charge a bonus for getting this call through?" retorted the operator. Whatever Burkett's reply to this might have been was never learned; her attention was diverted at that point. "We've just come in sight of the flow. It's about five hundred yards ahead. We'll get as close as seems safe, and I'll try to make sure whether it's really lava or just mud." "Mud? Is that possible? I thought there wasn't--couldn't be--any water on this planet!" "It is, and there probably isn't. The liquid phase of mud doesn't have to be water, even though it usually is on Earth. Here, for example, it might conceivably be sulfur." "But if it's just mud, it wouldn't hurt the ship, would it?" "Probably not." "Then why all this fuss about getting the tractors back in a hurry?" * * * * * The voice which answered reminded him of another lady in his past, who had kept him after school for drawing pictures in math class. "Because in my judgment the flow is far more likely to be lava than mud, and if I must be wrong I'd rather my error were one that left us alive. I have no time at the moment to explain the basis of my judgment. I will be reporting our activities quite steadily from now on, and would prefer that you not interrupt unless a serious emergency demands it, or you get a call from Eileen. "We are about three hundred yards away now. The front is moving about as fast as before, which suggests that the flow is coming only along this valley. It's only three or four feet high, so viscosity is very low or density very high. Probably the former, considering where we are. It's as black as the smoke column." "Not glowing?" cut in Zaino thoughtlessly. "_Black_, I said. Temperature will be easier to measure when we get closer. The front is nearly straight across the valley, with just a few lobes projecting ten or twelve yards and one notch where a small spine is being surrounded. By the way, I trust you're taping all this?" Again Zaino was reminded of the afternoon after school. "Yes, Ma'am," he replied. "On my one and only monitor tape."