On the 25th of July, Captain Bonneville struck his tents, and set out on his route for the Bighorn, at the head of a party of fifty-six men, including those who were to embark with Cerre.  Crossing the Green River valley, he proceeded along the south point of the Wind River range of mountains, and soon fell upon the track of Mr. Robert Campbell’s party, which had preceded him by a day.  This he pursued, until he perceived that it led down the banks of the Sweet Water to the southeast.  As this was different from his proposed direction, he left it; and turning to the northeast, soon came upon the waters of the Popo Agie.  This stream takes its rise in the Wind River Mountains.  Its name, like most Indian names, is characteristic.  Popo, in the Crow language, signifies head; and Agie, river.  It is the head of a long river, extending from the south end of the Wind River Mountains in a northeast direction, until it falls into the Yellowstone.  Its course is generally through plains, but is twice crossed by chains of mountains; the first called the Littlehorn; the second, the Bighorn.  After it has forced its way through the first chain, it is called the Horn River; after the second chain, it is called the Bighorn River.  Its passage through this last chain is rough and violent; making repeated falls, and rushing down long and furious rapids, which threaten destruction to the navigator; though a hardy trapper is said to have shot down them in a canoe.  At the foot of these rapids, is the head of navigation; where it was the intention of the parties to construct boats, and embark.

Proceeding down along the Popo Agie, Captain Bonneville came again in full view of the “Bluffs,” as they are called, extending from the base of the Wind River Mountains far away to the east, and presenting to the eye a confusion of hills and cliffs of red sandstone, some peaked and angular, some round, some broken into crags and precipices, and piled up in fantastic masses; but all naked and sterile.  There appeared to be no soil favorable to vegetation, nothing but coarse gravel; yet, over all this isolated, barren landscape, were diffused such atmospherical tints and hues, as to blend the whole into harmony and beauty.

In this neighborhood, the captain made search for “the great Tar Spring,” one of the wonders of the mountains; the medicinal properties of which, he had heard extravagantly lauded by the trappers.  After a toilsome search, he found it at the foot of a sand-bluff, a little east of the Wind River Mountains; where it exuded in a small stream of the color and consistency of tar.  The men immediately hastened to collect a quantity of it, to use as an ointment for the galled backs of their horses, and as a balsam for their own pains and aches.  From the description given of it, it is evidently the bituminous oil, called petrolium or naphtha, which forms a principal ingredient in the potent medicine called British Oil.  It is found in various parts of Europe and Asia, in several of the West India islands, and in some places of the United States.  In the state of New York, it is called Seneca Oil, from being found near the Seneca lake.