The American Cyclopaedia Entry for Iosco

IOSCO, a N. E. county of the southern peninsula of Michigan, bordering on Lake Huron and Saginaw bay, intersected by the Au Sable, and drained by the Au Grais river; area, 575 sq. m.; pop. in 1870, 3,163. The surface is nearly level, and partly covered with pine forests. In 1870 there were 11 saw mills, producing $646,151 worth of lumber, and 1 manufactory of ground plaster. Capital, Tawas City.