BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 11, pp. 503-510 August 10, 1900 



CONTACT METAMORPHISM OF A BASIC IGNEOUS ROCK 



BY ULYSSES SHERMAN GRANT 



{Presented before the Society December 30, 1899) 

 CONTENTS 



Page 

 Introduction 503 



Outline of local geology 503 



Keweenawan gabbro 504 



Contact metamorphism produced by the gabbro 505 



General character of the metamorphism 505 



Gray wacke-slate member of the Animikie 506 



Black-slate member of the Animikie 506 



Iron-bearing member of the Animikie 506 



The Keewatin 508 



The Archean * 509 



Summary 510 



Introduction 



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 lith- 

 ology 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. 



Outline of local Geology 



The triangular area of Minnesota lying north of lake Superior is under- 

 lain 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 

 Canadian boundary south to lake Superior one passes over rocks of the 



LXXII— Bull. Geol. Soc. Am., Vol. 11, 1899 (503) 



