"But what is this place, anyhow!" asked Nort. "It looks like the Yale bowl, but it never could have been built by man." "It wasn't," said Bud. "It's the crater of an extinct volcano. It has been filled up, with land-slides, probably, and the winds and the birds have brought grass seeds here, year after year, until it makes a regular corral for cattle. There's water, too, which isn't surprising. That's what it is, an old volcano crater. I heard there was one around here, but I never had time to look for it." "Yes, I've heard of it myself," admitted Slim, "but I didn't think it was like this. Let's have another look." Dick and Nort moved aside to give the foreman a place of advantage, and when he had looked through a spot where the crack was wider he said: "I see where they can get th' cattle out. Here, take a look, Bud," and Slim handed the ranch lad a pair of field glasses that had been brought along in case of emergency. They were of value now. "Down at th' far end, and a little to the left of centre," Slim directed Bud's gaze. "There's a sort of fence of trees piled up. That's th' entrance all right--or one of 'em." "You're right!" agreed Bud when he had taken a careful observation. "But is there more than one!" "Must be," said Slim. "The rustlers never drove th' cattle in away around _there_. They sent 'em in from _this_ end. Th' trail ends right here, an' it's here where th' rustlers drove th' cattle in." "But where?" asked Bud. "There isn't a sign of an opening!" "Because they closed it after them," went on the foreman. "I begin to see it now. There must have been a break in the wall of the old crater right about here. They drove th' cattle in an' it was an easy matter t' let some of th' dirt slide down an' fill it up again. Let's take a look with a view t' seein' if I'm right." It is easier to find a clue when you know just what you are looking for. And it did not take long for the experienced eyes of the cow punchers to discover where earth and shale from above had been recently dislodged and allowed to slide down to cover what must have been the same sort of natural opening into the side of the wall as that at the far end, closed by a fence of trees. This was to keep the cattle in without men being needed to ride herd. "Yes, it does look as if they'd taken 'em in here," said Bud, when it was found that the trail of the steers led to the foot of the crater wall, where all signs stopped. "If we had looked up a little, instead of sticking so close to the ground, we might have seen this clue before." "All in good time," observed Slim. "The question is, now, how can we get in there?" "It will be easy enough," suggested Nort. "All you'll have to do will be to enlarge the crack we looked through." "That's all right for us getting inside that crater," observed Dick, "but what about our horses? They can't scramble up there." "Then what can we do?" asked Bud. "Ride around to the other entrance?"