They justified their return in the following manner: First, they were satisfied that the Mississippi emptied into the Gulf of Mexico, and not into the Gulf of California, nor into the Atlantic Ocean in Virginia. Second, they feared a conflict with the Spaniards, who occupied and claimed the Gulf coast. Third, they feared the Indians of the lower Mississippi, for they used firearms and might oppose their further progress south. Fourth, they had acquired all the information they started out to obtain.
And so, on the 17th of July, 1674, they turned their faces homeward. They had been just two months, from May 17th to July 17th, on their journey. They had traveled more than a thousand miles. They had faced all forms of danger and had undergone all manner of hardships. Their provisions had been obtained en route. France owes them a debt of gratitude which will never be fully paid. Indeed not only France, but the world is their debtor. Nothing of interest occurred on their return journey until they reached the mouth of the Illinois River. Here they were told by some Indians that there was a much shorter route to Green Bay than by way of the upper Mississippi and the Wisconsin and Fox portage. This shorter route was up the Illinois River to the Chicago portage and thence along Lake Michigan to Green Bay. Marquette and Joliet proceeded up the Illinois River. When passing by Peoria Lake they halted for three days. While here Marquette preached the gospel to the natives. Just as Marquette was leaving they brought him a dying child which he baptized. When in the vicinity of Ottawa, they came to a village of the Kaskaskia Indians. Marquette says there were seventy-four cabins in the village and that the Indians received them kindly. They tarried but a short time and were escorted from this point up the Illinois and over the Chicago portage by one of the Kaskaskia chiefs and several young warriors.
While in the village of the Kaskaskias, Marquette told the story of the Cross to the natives, and they were so well pleased with it that they made him promise to return to teach them more about Jesus. Marquette and Joliet reached Green Bay in the month of September, 1673. Probably they both remained here during the ensuing winter. In the summer of 1674, Joliet returned to Quebec to make his report to the governor. On his way down the St. Lawrence, his boat upset and he came near losing his life. He lost all his maps, papers, etc., and was obliged to make a verbal report to the governor.
Source: A Standard History of Champaign County, Illinois, by J. R. Stewart, published by The Lewis Publishing Company, Chicago And New York, 1918.