skeleton! Its bleached bones wore the same peculiar blue glow. Patti, accustomed to seeing the bones of men in her father's laboratory, as well as in the jungles, did not shudder at the sight. Instead she looked at it curiously, its grinning jaws facing her, sightless sockets staring wide.
Norton exchanged glances with the scientist and then stepped back.
"That's a warning of some sort, Doctor Marsden," he said quietly, "apparently placed there to stop anyone from going farther on this trail.
"No doubt," replied the scientist. "I've seen such warnings before in Brazil. I always found it unhealthy to ignore them. But we've got to go on. If Indians hung that skeleton up there, our number and rifles ought to hold them in their places."
"It proves that we're not alone here," interjected Bob anxiously. "What do you suppose makes those bones glow as they do, as everything does here, Doctor?"
"The place is fairly salted with radio-active elements, Bob," said Dr. Marsden quietly. "Notice how warm the ground feels under your feet?"
Bob nodded.
"Well," continued Dr. Marsden, "since there seems to be no sun here to warm it, the earth must contain radium. Without radium or other radio-active minerals present to radiate warmth, this place would have a temperature far below freezing."
"There might be some volcanic activity below," suggested Bob.
Dr. Marsden shook his head thoughtfully.
"I do not believe so," he said. "If there was activity, we would doubtless smell carbon or sulphur compounds."
As if that settled the matter, they gazed upon the skeleton again. Then Bob unsheathed his machete, stepped ahead of Norton and slashed the thong. The skeleton crashed aside with a dismal rattle of bones. They went on past it.
CHAPTER IV
LIEUTENANT Bob Allen now in the lead, was tensely alert as he led the bedraggled castaways along the forbidden trail. After another good hour of steady plodding they noticed an abrupt change in the topography and in the character of vegetation. The ground began to descend suddenly. The trees and vegetation grew more twisted as if in torture. There seemed a certain transparency in them now that created no little interest and wonder in Dr. Marsden. The others wondered at it, too, but only the scientists appeared to know hypothetically the reason for the transparency.
The presence of radium became more and more conspicuous in every living thing as they proceeded. Because of its quantity or oxidation, Dr. Marsden thought, the radium seemed to have the effect of a powerful X-ray on plant life. But he was no master of physics, metallurgy or chemistry, and while he had his theories of the reaction of radium oxide on matter, he was not scientifically fitted to get at the bottom of the strange mysteries confronting him and to offer a concrete solution.
The descending ground, however, gradually became steeper until the going presented a problem. It was like climbing down a steep mountain now. But far below, half-hidden in a blue haze that shimmered like heat-waves on a flaming desert, lay an expansive valley. The trail, in places partly overgrown and never more than two feet wide, wormed its way down the precipitous slope toward it. So steep did it become eventually, that Bob remarked to Patti, that only a mountain goat could traverse it with any degree of safety.
Patti smiled grimly. Norton made an attempt to help her down a particularly steep incline and bungled her descent. Bob caught her arm as she started to slide. Regaining her footing, she gave Norton an indifferent smile. He shrugged his broad shoulders and went on, leaving her to her own destinies. She held her own, however, and managed the grade without further assistance.
She realized suddenly that she was very tired and weary. She felt a desire to throw herself on the ground and go to sleep. But gamely she fought off her feelings of exhaustion. After what seemed ages they attained smoother ground at the base of the mountain. How far down they had climbed from the lake they could not know exactly. Bob judged for himself that they must have dropped at least four or five thousand feet.
A wall of matted brush confronted them at the bottom. The narrow trail wound about its edge, then finally entered the brush. A hole at some time or other had been hacked through the brittle undergrowths like a tunnel. Though Patti could have walked through it without bending over, she was nevertheless compelled to do so to escape catching her hair in low-hanging twigs or the stinging dust that frequently was shaken upon them by their contact with the growths. Bob and the others were compelled to stoop laboriously as they went on. Weighted down by their packs and rifles, this made the going all the harder for them. Yet none complained, except Norton who swore under his breath.
He walked directly behind Bob. Dr. Marsden followed him. Then came Patti, the crew behind her. It would have been a simple matter there in the semi-darkness of the underbrush, for Norton to carry out his threat by shooting point blank at Bob. But he knew that Dr. Marsden was on the alert; that one significant move in that regard might cause the scientist to swing into action with his own gun. So Norton wisely betrayed no signs of hostility toward the man he hated.
"I'll gamble this tunnel wasn't made by big men," Bob remarked over his shoulder. "Indians could hardly have made it."
"That's what I’ve been thinking," responded the scientist. "It's not an Indian trail. An Indian could not walk upright in it and they detest stooping, because its hard work. They'd go miles to get around this area of brush."
"There may be a tribe of pygmies living here," interjected Norton with some sarcasm. "Haven't you thought of that, Mister Allen?"
Before Bob could reply, his rifle suddenly flew from his grasp. The ground beneath him seemed to sink downward. He gave a yell of warning and vanished from the rtail, dropping into a hole like a plummet.
The party stopped short. Norton gave a guttural grunt of surprise, when he saw that Bob had vanished from view with a crashing of brush. Patti gave a muffled cry and went on. Her father held her back.
