This paper summarizes the results of the Geological and Natural History Survey of Minnesota as to the sequence of Precambrian rocks in and near Minnesota.
Eleven formations are noted, and exposures are described to show a well-established sequence. The division planes between groups are also established by four well-studied and described unconformities, two large and two smaller. Other Precambrian formations in the State are more isolated and only tentatively correlated with the main sequence. There have been suggestions also of other unconformities, but the evidences for these are considered doubtful.
The principles and methods of correlation in the Precambrian are briefly noted and applied. Most valuable are the ages determined by radioactivity, the characters of the zircons in large intrusives, the petrographic peculiarities of the iron-formations, and the sequence of beds.
The sequence finally indicated suggests satisfactory correlations of most formations in the State, but only tentative correlations with other series as remote as those north of Lake Huron, the type locality of “Huronian”. Most other well-studied series are not so complete as that in and near Minnesota, and more work is needed to make correlation satisfactory.