50 [ I'. S. GRANT — CONTACT METAMORPHISM OF BASIC IGNEOUS ROCK 



following ages in the order named : Archean, Lower Huronian (or Kee- 

 watin), Upper Huronian (or Animikie), and Keweenawan. These series 

 are separated by unconformities. The first two series frequently have 

 been closely folded, while the last two dip toward the south-southeast at 

 angles of from 5 to 20 degrees. 



The Archean consists in the main of greenstones and of granites, more 

 or less schistose. The former were originally basic igneous rocks, and in 

 most cases they have not as yet been carefully separated from the over- 

 lying Keewatin, which also contains masses of greenstone. Granites, 

 intrusive into the Keewatin, also occur. 



The Lower Huronian (or Keewatin) is composed of a variety of sed- 

 imentary and igneous rocks, the most numerous of which are conglom- 

 erates, graywackes, jaspilytes, greenstones, and slates and schists of 

 several kinds. 



The Upper Huronian (or Animikie) rests unconformably upon the 

 older rocks, and along the contact with the gabbro has not been sub- 

 jected to the intense dynamic action which has folded and sheared these 

 older rocks. The dip of the Animikie averages about 10 degrees toward 

 the south or south-southeast, but where it disappears under the gabbro 

 the dip is commonly much steeper. The Animikie is separable into 

 four members, in ascending order, as follows : 



1. The quartzite member. 



2. The iron-bearing member, in which the important iron deposits of 

 the Mesabi range occur. 



3. The black-slate member, which is composed essentially of black, 

 carbonaceous, frequently very fissile, slates. 



4. The graywacke-slate member, consisting of black to gray slates and 

 fine grained graywackes, with some flinty slates and slat} 7 quartzites. 



The lower member is lacking in the district where the gabbro occurs. 



The Keweenawan consists at its base of a mass of gabbro, which is 

 frequentty associated with more acid rocks of somewhat later date- 

 Above these is a series of basic igneous rocks, with the uppermost parts 

 of which some sandstones and conglomerates are intercalated. 



Keweenawan Gabbro 



The gabbro occupies a roughly crescentic area which reaches lake 

 Superior at its western end, but elsewhere is several miles north of this 

 body of water. The crescent is some 125 miles in length, and the total 

 surface area underlain by the gabbro is*approximately 1,000 square miles. 

 It is therefore a mass of considerable size. 



The gabbro is a coarse grained aggregate of plagioclase, which is near 



