The present cave was one of the few exceptions to this rule. The rock itself was not porous enough for travel, but seismic strains had produced a network of miscroscopic cracks part way into the mass which permitted slow progress, if the traveler had persistence.
Derrell had seen caves before from a distance, but the thing he was watching now had never occurred within his memory or knowledge. The upper level of the bubble was just at the top of the igneous layer in which it had formed; the rock above was sedimentary. Between the two layers a thin sill was gradually making its way from a pool of magma a few miles distant—a pool that was being fed by energy from sources far below, and outside the bounds even of Derrell’s knowledge. It was more than likely that some day this sill might grow to the proportions of a laccolith, in view of the nature of the rock above; but this was not the scientist’s concern at the moment. The advancing magma was approaching the bubble, and he wanted to see what effect the trapped, high-pressure gas in the “empty” space would have on the molten rock. It was fortunate that this was occurring just here; the endless, tiny seismic shocks from the southwest made things clearly visible throughout the region. It would have been extremely dangerous, with the Asian savages filtering through the neighboring strata, if the investigators had had to produce sounds of their own in order to further their research.
Derrell had a mental picture of what would occur when the molten rock reached the bubble, but like any good scientist he was not allowing it to influence his observing technique. He intended to see everything that happened; and his attention was so completely centered on that particular volume of rock that the arrival of one of his assistants, who had been on a short leave to the nearest of the frontier cities, failed to distract him in the slightest. The assistant himself forbore to interrupt, though he had news that he knew would be of interest; for the magma was very close now to the bubble. Like the chief scientist, the newcomer had a mental picture of the hot fluids simply reaching the cavity, flowing around its walls, and gradually filling it from edge to center. Like Derrell, his idea of the general nature of gases was too sketchy to permit him to realize that, at the very least, the vapor in the bubble must dissolve in the inflowing rock before his picture could be carried through; and like his chief, he had no conception whatever of another force that would also operate. No living member of their race had ever had a good look at a fluid that was not in a confined space; they had never seen a free liquid surface. Their experience was about to be enlarged.
There would be no point in guessing who was the most surprised by what actually happened, but there was no doubt about which of the observers adjusted himself first. Derrell was paralyzed for just an instant as the first drops of fluid reached the opening of the cavity—and shot straight across it to the other side!— buthe noted carefully and precisely how more of the magma followed. The drops became a stream, and gradually a pool of the stuff came into being against the side of the bubble opposite the opening. The sides of the pool not in contact with the walls of the cavity seemed to want to form a plane surface, but the stream that was adding to its volume gave rise to disturbances which spread from the point of impact in all directions over the surface— waveswhich none of the watchers had ever seen or imagined, and which held even the sentries’ attention to a degree which might have proven disastrous. Not until the bubble had filled completely with molten rock did anyone move, speak, or even think of anything but what was happening a few hundred yards away; and even then most of the team waited for Derrell to express an opinion. He, regarding his assistants as students to be guided by suggestion rather than laymen who might be impressed by spot conclusions, opened his comments with a question.
“Could the ordinary pressure on that liquid account for its behavior?”
“Not completely.” The answer came promptly from one of the team.
“Why not? Pressure can force liquid between rock layers, and even into rock pores; why could it not send a stream across a space where there is no resistance?”
“It could, I suppose; but I fail to see how ordinary rock pressure could keep one side of that growing pool flat when the rock was not actually touching it. That would seem to call for some invisible substance pressing on that particular surface—a substance not only invisible, but able to permit the stream of rock to pass toward the new pool but not away from it. I find such a substance hard to imagine.”
“So do I. Your objection to rock pressure also seems valid—unless someone else can see a way?” He paused for a fair interval, but if any of the assistants had ideas they were not sufficiently formulated for expression. “It would seem, then, that some force with which we are unfamiliar is involved. That means that all the data anyone may have is possibly relevant. Karpor, list the material you have observed which you think might help.”
The student responded at once.