The widespread Lower Mississippian limestones of the Madison group were deposited in a basin extending eastward from the Cordilleran trough, bounded on the north in Alberta and Saskatchewan by a positive element related to the Sweetgrass arch, and on the south by positive elements of northern Colorado and northwestern Nebraska. The limestones may be divided into two persistent formations: the very massive Mission Canyon limestone overlying the well bedded Lodgepole limestone. The Lodgepole is divisible into an upper member, the Woodhurst, largely limestone with thin shale partings, and a lower member, the Paine, with a much larger percentage of shale. Petroleum-reservoir zones, largely confined to the Mission Canyon, appear to have been controlled by five factors: solutio , dolomitization, sandy zones, reworking of oolitic and granular material, and fracturing. Insoluble-residue analysis indicates that the formations and members of the group may be recognized by that means in a large area and that zones within formations may be determined in limited areas.