"'The river must therefore turn much to the south between this and the rocky mountain to have permitted Mr. Fidler to have passed along the eastern border of these mountains as far south as nearly 45 deg. without even seeing it.... Capt. Clark says its course is S. 29 W. and it still appeared to bear considerably to the W. of South.... I think therefore that we shall find that the Missouri enters the rocky mountains to the North of 45 deg. We did take the liberty of placing his discoveries or at least the Southern extremity of them about a degree farther North ... and I rather suspect that actual observations will take him at least one other degree further North. The general course of Marias river ... is 69 deg. W. 59'.' "Lewis also figured that Fidler in his map showed only small streams coming in from the west, 'and the presumption is very strong that those little streams do not penetrate the Rocky Mountains to such distance as would afford rational grounds for a conjecture that they had their sources near any navigable branch of the Columbia.' He was right in that--and he says those little creeks may run into a river the Indians called the Medicine River. Now that is the Sun River, which does come in at the Falls, but which Lewis had never seen! "Again, the Minnetaree Indians had told him, in their long map-making talks at the Mandan winter quarters, that the river near the Falls was clear, as he now saw this stream. The Minnetarees told him the Missouri River interlocked with the Columbia. And as he was now straight west of the Minnetarees, he figured that when they went hunting to the head of the Missouri, as they had, they couldn't have passed a river big as this south fork without mentioning it. And the Indians said that the Falls were a 'little south of the sunset' from the Mandans--and Lewis had his latitude to show he was still on that line and ought to hold to it. "Lastly, he reasoned that so large a river must penetrate deeply into the Rockies--and that was what he wanted. He knew it could not rise in dry plains. So, relying on his Minnetarees and his horse sense, and not on Mr. Fidler, Lewis refused to go any farther north, because he could not figure out there a big river penetrating into the Rockies. He was absolutely right, as well as very shrewd and wise. "Now, reasoning at first shot, the _voyageurs_ would have gone up the Marias. Cruzatte especially, their best riverman, was certain the Marias was the true Missouri. They would then maybe have met the Blackfeet and would never have crossed the Rockies; which would have meant failure, if not death; whereas this cold-headed, careful young man, Meriwether Lewis, by a chain of exact reasoning on actual data, went against the judgment of the entire party and chose the left-hand fork, which we know is the true Missouri; and which we'll find hard enough to follow to its head, even to-day.