Harvey was called at two o’clock in the morning, and he posted himself as sentinel under a small tree that grew near the shelter-tent. He had become somewhat accustomed to being rudely awakened and to being alone while the others slept, and now that an attack by Indians was improbable, and it was no longer necessary to strain his sense of hearing that he might note the slightest sound, the novelty of the situation appealed to him.
This night the moon in its third quarter shone from out a cloudless sky, and at the altitude of the great intermontane valley in which they rested, the rays were brighter than at points nearer the sea level, so the river bank and the open country were visible with nearly the distinctness of day.
As the boy walked a few times back and forth, a rifle on his shoulder, then paused for a short rest under the tree, he puzzled his brain to account for their not having found the second white rock. He believed implicitly in the truth of all that Huayno had said, and was confident that not far from where he stood great riches were stored in the ground.
But could they ever locate the mine? It would be a task of years to demolish all those mounds and ascertain which hid the entrance to the old workings; and should it be attempted, others must learn what they were doing on the banks of the Marañon, others would flock to the place with picks and shovels, and among these others some one or two might first find the store of yellow metal.
Thus cogitating he walked closer to the river and stood beneath the great white rock, which shone resplendent in the moonlight, glistening and seeming to be translucent. Studying the strange geological formation attentively, he noticed for the first time that only the side facing up stream and the side facing the woods were white; those facing down stream and the opposite shore were much darker, almost a slate color. This peculiarity had not been remarked, because no member of the party had gone farther down stream. The boy also saw that the rock was several feet from the river and that its lower portion, where the water washed, had turned this same slate color.
He paced slowly back to the tree, meditating on these observations, and endeavoring to solve the reason for the varying of the physical features of the unique landmark. In the midst of this his mind strangely reverted to the time of a dinner party that had been given at his father’s home in Chucuito about six months before, and try as he might he could think of nothing else than this entertainment and the people who were present; then of the conversation that had occurred—and the moment the mind cell that contained the impression left by that conversation opened, he had the solution of the problem which confronted them.
At this dinner Don Isaac Lawton, editor of the South Pacific Times, had been asked to explain the absence of rain on the Peruvian coast-line. He had done so in these words:—
“The absence of rain on the coast is caused by the action of the lofty uplands of the Andes on the trade-wind. The southeast trade-wind blows obliquely across the Atlantic Ocean until it reaches Brazil. By this time it is heavily laden with vapor, which it continues to bear along across the continent, depositing it and supplying the sources of the Amazon and the La Plata. Finally, the trade-wind arrives at the snow-capped Andes, and here the last particle of moisture is wrung from it that the very low temperature can extract. Coming to the summit of that range, it rushes down as a cool and dry wind on the Pacific slopes below. Meeting with no evaporating surface, and with no temperature colder than that to which it is subjected on the mountain tops, this wind joins the south trades and reaches the ocean before it becomes charged with fresh moisture.”
Harvey, recalling this conversation, for it had been imprinted upon his mind, because it was the first explanation he had heard of this Pacific coast phenomenon, began to reason that if the trade-winds blew in a certain direction over Brazil and in a certain direction on the coast, there was undoubtedly a regularity of the wind currents in this intermontane valley. He had noticed since leaving Huari that what breeze stirred, blew in their faces; therefore the general direction of the wind was up stream, or toward the southwest.
That being true, the reason why a portion of the great white rock had turned a slate color was evident—it was weather-stained, and the remaining portion, sheltered from the winds, retained its lustre. At this stage in his reflections he recalled a sentence from his geology: “Alabaster is soluble to a certain extent in water.”