"So that must be the Rock over yonder. We're below the canyon, and below the Wisdom, and below the Philanthropy, and below the end of the railroad, and in the third valley. Besides, look at it. Just as sure as Sacagawea was about it!" "You're right," said Billy. "That's the Point of Rocks, as it's called now." They made down to the edge of the valley and went into camp across from the great promontory which so long had served as landmark in all that country. That night all of them forded the river horseback and rode close to the historic point. Jesse, who was prowling around on foot, as was his habit, closely examining all he saw, suddenly stooped, then rose with an exclamation. "See what I've found!" said he. "What is it--a gold nugget?" asked his uncle. "No. An arrowhead. Funny one--looks like it was made of glass, and black glass at that." Uncle Dick examined it closely. "Jesse," said he, "that's one of the most interesting things we've run across on this whole trip. Did you know that?" "No. Why?" "You wouldn't think that arrowhead was going to take you to the true head of the Missouri, and to good fishing for trout and grayling, would you?" "Why, no! How's that?" [Illustration: JESSE SUDDENLY STOOPED, THEN ROSE WITH AN EXCLAMATION] "I'll tell you. That's an obsidian arrowhead. The Bannacks and Shoshonis got that black, glassy stuff at one place--the Obsidian Cliff, in Yellowstone Park! Those old trails that Lewis saw to the south were trails that crossed the Divide south of here. They put the Indians on Snake River waters. These tribes hunted down there. They knew the head of the Red Rock. They knew the head of the Madison. They knew the Gibbon River, and they knew the Norris Geyser Basin, up in Yellowstone Park. It's all right to say the Indians were afraid to go into Yellowstone Park among the geysers, but they did. They knew the Obsidian Cliff--close by the road, it is, and one of the features of the Park, as it now is. "It's a far shot that arrow will carry you, son. It will show you more of these Indian trails than even Lewis and Clark ever knew. Of course, they didn't want to go south; they wanted north and west, because they knew the latitude and longitude of the mouth of the Columbia River. They knew that was northwest. They knew any water they got on, once over the Divide, would run into the Columbia, and they could see the Rockies, just on ahead to the west. As Billy has said, the Indian girl always was telling them that her people lived along in here. An obsidian arrow meant nothing to them. But it meant much to later explorers to the south of here." "It's a good specimen he's got," said Billy, looking it over. "The Indians liked to work obsidian; it would cleave so sharp and clean. I thought they had them all picked up, long ago. Up in the Shoshoni Cove they found a good many, first and last. All this was their hunting ground. A little over the Divide it gets awfully rough, and not much game."