198 C. K. LEITH 



group consists of volcanic fragmental material, in part deposited in 

 water. All of these clastic rocks now stand in nearly vertical positions, 

 with a strike a little north of east. 



Sharply marked off from the clastic rocks are four types of igneous 

 rocks, hornblende-porphyrite, granite, diabase, and gabbro. The gran- 

 ite is divisible into two types, ordinary granite, and granite-porphyry, 

 in both of which the ferro-magnesian constituent is almost exclusively 

 pyroxene and the predominating feldspar anorthoclase. The origin of 

 the granite is truly eruptive; having broken through the surrounding 

 elastics ; the rock is not formed, as held by N. H. and A. Winched, 

 from the recrystallization in situ of the sedimentaries of the region. 



The slate formation, the green schist, and the volcanic tuff belong 

 to the Keewatin, the Minnesota equivalent of the Lower Huronian. 

 The conglomerate contains pebbles, many of which are similar to some 

 of the Keewatin rocks, and it seems to belong to a newer series, 

 although as yet no unconformity between the conglomerate and other 

 rocks has been discovered. Following Lawson, it is believed that the 

 conglomerate is a part of the Keewatin, probably separated from the 

 lower part Of that series by an unconformity, and that it is much older 

 than the Animikie. However, the question whether the elastics belong 

 to one or two series is as yet open. 



The porphyrite and the granite are of Keewatin age. The por- 

 phyrite is regarded as contemporaneous with the deposition of volcanic 

 tuff and green schist, and the granite is believed to date from the fold- 

 ing of the Keewatin. The age of the diabase dikes is not known ; 

 they are perhaps contemporaneous with the great diabase intrusions in 

 the Animikie. The gabbro is of early Keweenawan age. 



Grant 1 sketches the geology of the eastern end of the Mesabi iron 

 range in Minnesota, including T. 64 N., Rs. 3 and 4 W., and parts of 

 Rs. 2 and 5 W., with some adjacent portions of Ontario. The rocks 

 can be separated into three divisions. The chief one of these is the 

 Animikie series, containing the iron bearing rocks of the Mesabi range. 

 Older than the Animikie is a series of granites, greenstones both massive 

 and schistose, conglomerates, slates, and other clastic rocks, called the 

 pre-Animikie. Younger than the Animikie are some diabase sills and 

 the great gabbro mass of northeastern Minnesota. 



1 Sketch of the geology of the eastern end of the Mesabi iron range in Minnesota, 

 by U. S. Grant: Engineers' Year Book, Univ. of Minn., 1898, pp. 49-62. With sketch 

 map. 



