"This ridge," I said, "is about three feet higher than where we landed the ship. I bet the hole in the rock and the hole in this cactus are on the same level."
Gonzales said slowly, "This was not done all at once. It is a result of periodic attacks. Look here and here. These overlapping depressions along the outer edges of the hole—" he pointed—"on this side of the cactus. They are the signs of repeated impact. And the scallop effect on this side, where whatever made the hole emerged. There are juices still oozing—not at the point of impact, where the plant is desiccated, but below, where the shock was transmitted—"
A distant shout turned us around. Burton was at the rock, beside the ship. He was bending down, his eye to the far side of the mysterious hole.
He looked for another second, then straightened and came toward us at a lope.
"They line up," he said when he reached us. "The bottom of the hole in the cactus is right in the middle when you sight through the hole in the rock."
"As if somebody came around and whacked the cactus regularly," Janus said, looking around warily.
"To keep the line of sight through the holes clear?" I wondered. "Why not just remove the cactus?"