The crystalline signal came in a rippling flood. He spun the dials. The sound ceased, and the pointing rods glowed with an aura of amber light at their tips. Swift and startling answer came from deep within the heart of the cliff, a mighty note of sonorous beauty like the violent plucking of a string on some colossal bass viol. So powerful was the timbre of the pulsing sound that the entire side of the mountain seemed to vibrate in harmony with it. Layroh snapped off the apparatus and the sound ceased. Carefully searching until he found a certain spot on the cliff face, he stepped close to it and unlimbered the nozzles of the silver cylinder. Foster noted that at the place selected by Layroh there was a five-foot-wide stratum of slightly lighter-colored rock extending from the sand to a point high up on the cliff face. From the metal nozzles of the cylinder there spurted a broad beam of dead black. There was a searing flash of blue-white flame as the black beam struck the cliff face. There followed a brief second during which the rock melted into nothingness in the heart of that area of blue radiance. Then the stabbing beam bored steadily on back into the cliff like the flame of a blow torch melting a way through a block of butter. Layroh adjusted the nozzles until the black beam was a solid shaft of opacity seven feet in height and nearly five in width. The hole in the cliff became a tunnel from which blue radiance surged outward in a shimmering mist as the black beam steadily bit deeper into the rock. * * * * * "Follow me," Layroh ordered the men, "but do not approach too close." He stepped forward and entered the mouth of the tunnel. Shaken by the spectacular thing occurring before their eyes, yet, driven by curiosity as to what might lie at the end of that swift-forming tunnel, the men came crowding obediently after him. A moment later they were within the passage, stumbling dazedly forward through the billowing fog of bluish radiance. There was an odd, almost electric, tingle of exhilaration in that radiant mist as it surged about their bodies. Fragments of almost-forgotten scientific lore flitted through Foster's brain as he groped for a clue to the action of the strange ray. Not quite complete disintegration of matter, but something very close to it--probably the transformation of matter into radiant energy, an ingenious harnessing of the same forces that are forever at work in the cosmic crucibles of the universe's myriad suns. The action of the black ray was amazingly rapid. They were forced to hurry forward at a fast walk to keep their distance behind Layroh. The vertical stratum of lighter-colored rock continued straight back into the heart of the mountain. It apparently served as a guide. The color of the blue flame-mist changed perceptibly whenever Layroh allowed the black ray to stray into the rock at either side of it. * * * * *