In volumes 4 and 5 of the Final Report of the Geological and Natural History Survey of Minnesota, and in some of the annual reports of the same survey, mention is frequently made of certain peculiar crystalline rocks which occur along the northern edge of the great gabbro mass of northeastern Minnesota. The rocks in question furnish one of the most interesting cases of metamorphism of a series of rocks of varied lithology by the contact effect of a basic igneous mass. It is the object of this paper to present in outline an account of the phenomena here seen.
The triangular area of Minnesota lying north of lake Superior is underlain by rocks of pre-Cambrian age. In general these are disposed in belts, which trend east-northeast and west-southwest. Those of greatest age are toward the north, so that in traversing this district from the . . .