Grout et al. (1951) concluded that the Precambrian formations of Minnesota were deposited during three eras which they named Earlier, Medial, and Later; and James (1955) proposed Lower, Middle, and Upper divisions for the Precambrian of northern Michigan. These threefold classifications apparently are meant to replace the dual classification so long in use in the Lake Superior region and although possibly not so intended imply by the use of the word Precambrian that they are applicable to the whole Canadian Shield. Although there is much too little information as yet for certainty, probably not more than one recognizable major unconformity is sufficiently extensive in the shield to permit the division of Precambrian time into more than two eras by means of stratigraphical investigation.
Only a tentative dual classification of the rocks of the shield according to their relationship to the great unconformity beneath the (Huronian) Cobalt Series on Lake Timiskaming has been attempted. Formations beneath this unconformity in the Timiskaming and Grenville subprovinces and less positively in the north or northwest subprovince are classed as Archean (Early Precambrian), those above as Proterozoic (Late Precambrian). Rocks in the northern part of the shield are either assigned on lithological and structural evidence to the Archean or Proterozoic or are unclassified under the designation Archean and/or Proterozoic. In places, the Archean classification of formations has been confirmed by physicochemical age determinations of minerals.
The Canadian Shield is divided by geographical and geological barriers into provinces and subprovinces. The subprovinces delimit the territory to which direct serial correlations can be extended with reasonable certainty. At the present the use of most serial names in more than one subprovince isunwarranted.