The rocks described in the following pages occupy the entire area of southeastern Minnesota, some 13,200 square miles in extent. They stretch eastward from a straight line between Mankato and Hinckley to the state of Wisconsin, and from Chengwatona southward to Iowa.
The periods of geologic time represented by these formations are three, viz, Cambrian, Silurian and Devonian. That portion of the Cambrian exhibited is the upper, of the Silurian the lower, and of the Devonian so thin a layer is present and so few fossils occur in it that we cannot assign the rocks to any division of that group, but suppose them to belong near the middle.
These Paleozoic rocks are underlain by the Archean and Algonkian, and lie beneath patches of Cretaceous and a covering of Quaternary débris save in that extreme southeastern corner included within the “driftless area” of Chamberlin.*
The lowest rocks considered in . . .