We have now followed the career of Captain La Barge through the various experiences of youth and early manhood until he is finally settled in the business of his subsequent life--the navigation of the Missouri River. It is therefore a proper time to consider the nature of that business, its features of peculiar interest, and its relation to the growth of the western country. This is the more important because it is a phase in the development of that country which has permanently passed away, and in the general mind is already buried in oblivion. Yet for fully a hundred years the history of the Missouri River was the history of the country through which it flowed. Its importance no one to-day can comprehend. Now the railroad has made accessible almost every section of the country. Then there were no railroads to speak of west of the Mississippi, nor, for that matter, any other roads worthy of mention. The river was the great, and almost the only, highway of travel and commerce. Everything was done with reference to it. Commercial posts and military garrisons were established; expeditions were undertaken, and all business operations were carried on with careful reference to this mighty stream, which descended from the distant mountains to the very heart of the continent and thence to the sea, whence the road was open to every quarter of the globe. But now its influence upon the growth of the western country has ceased to exist. The mighty river, which was once alive with steamboats and other craft, from the Great Falls to its mouth, cannot boast a single regular packet. In the most absolute sense its glory has departed, and not a trace is left to remind the modern observer of its former greatness. In the following descriptions, therefore, we hope to be serving the true purpose of history, in gathering together for preservation some interesting features of a type of our frontier life which has long since run its appointed course. Of all the rivers on the globe the longest is the Missouri-Mississippi. On the summit of the Rocky Mountains, above the upper Red Rock Lake, some forty miles west of the Yellowstone National Park, and directly on the boundary between the States of Montana and Idaho, Jefferson Fork of the Missouri, finds its source. From this point, by a continuous water course to the Gulf of Mexico, the distance is 4221 miles. The river is formed by the confluence of three fine mountain streams which unite at a point about fifty miles south of Helena, Mont. They were named by their discoverers, Lewis and Clark, the Jefferson, Madison, and Gallatin rivers, in honor of the national administration which set on foot the expedition of these explorers. Two of these streams rise in the Yellowstone National Park, and the other, as we have seen, a little distance to the westward. [Sidenote: THE YELLOWSTONE.]