At that point it is said that he joined a government expedition around the Great Lakes which purpose has not been determined. Edson I. Carr's history of Rockton, published in 1898, relates a romance that Mack learned from traders at Green Bay about Illinois and possible opportunities there, so Mack traveled to the Rock River to the present site of Janesville and then traveled south into Illinois. At a Winnebago village at the present site of South Beloit, he learned of another village farther south located at the present site of Hononegah Forest Preserve, however legend says that Mack took the wrong path and ended up some forty miles to the southwest at a Winnebago village at the present site of Grand Detour in Ogle County. There he met a trader named Lasallier (perhaps his full name was Pierre St. Clair dit Lasallier), an old trading veteran who had operated trading posts on the Illinois and Rock Rivers since 1793. It is much more probable that the truth was that Mack had already known that Lasallier was there and had intended to work for him as a clerk, a usual practice for trading companies.