Probably no other exposures of slates are more favorable for a study of contact metamorphism than those of northeastern Minnesota. Two large batholiths of somewhat differentiated granite, some smaller granite and syenite stocks, and one of the largest known masses of gabbro intrude the slates at many places and modify them notably over wide belts. The effects of each are characteristic. Furthermore, the slates, though variable, are remarkably uniform in their main development—special phases are local and not likely to confuse the study of the metamorphic products. Finally, it has been possible to gather some 30 analyses and to examine more than 200 thin sections of the slate formations and the rocks formed by contact action. Few American districts have been thus tested.
Opportunity to study these series has come to the writer in the course of several summers’ work for the Minnesota Geological Survey, and . . .