We yesterday returned to St. Louis, and after a brief halt came on here. As our journey back to St. Louis was in the daytime, we had an opportunity of seeing the very interesting country which we passed on Saturday in the dark. The most remarkable feature of the road was crossing the Osage within 200 or 300 yards of its confluence with the Missouri. It is about 1,200 feet broad, and we saw in it one of those beautiful steamboats which give so much character here to the rivers. The Osage is navigable for these large boats for 200 miles above this place. We passed various other rivers, among others the Gasconade, at a spot memorable for a terrible catastrophe which happened on the day of the opening of the railway, when the first bridge which crossed it gave way as the train was passing, and nine out of thirteen cars were precipitated into the bed of the river; thirty people, chiefly leading characters of St. Louis, were killed, and many hundreds desperately hurt. We have little more to say of St. Louis, as the museum was the only public building we visited. The great curiosity there is the largest known specimen of the mastodon. It is almost entire from the tip of its nose to the tip of its tail, and measures ninety-six feet in length. We left St. Louis, and were glad to escape for a time at least out of a slave state. The "institution" was brought more prominently before us there than it has yet been, as St. Louis is the first town where we have seen it proclaimed in gold letters on a large board in the street, "Negroes bought and sold here." In the papers, also, yesterday, we saw an advertisement of a "fine young man" to be sold, to pay a debt. We took our departure in the Alton steamboat, in order to see the first twenty-four miles of the Upper Mississippi, and the junction of that river and the Missouri, which takes place about six miles below Alton; both rivers, however, are very tame and monotonous, and it was only as we were reaching Alton, that the banks of the Mississippi assumed anything like height. Alton itself stands very high, and as it was getting dark when we arrived, the lights along the hills had a fine effect. We are told it is a pretty town, but it was dark when we landed, and we had to hurry into the train that brought us to this place. The steamboat in which we went up the river was a very fine one, but not at all fitted up in the sumptuous manner of our Newport boat. Papa paced the cabin, and made it 276 feet long, beyond which there was an outside smoking cabin, and then the forecastle.