REVIEWS 85 



Near its contact with the underlying rocks, both the Animikie and 

 Keewatin series, there are various altered rocks which can be con- 

 nected in places with the gabbro and in places with the underlying 

 rocks. To these altered rocks the term muscovadyte has been applied. 

 It includes the various so-called peripheral phases of the gabbro. 



On the southern and eastern border the gabbro is penetrated by and 

 penetrates in a confused manner the red rock, with which it alternates 

 both structurally and areally. It is believed to have resulted from the 

 metamorphism by the gabbro of the Animikie, and perhaps earlier 

 fragmentals. 



As the granites of the Archean are believed to have resulted from 

 the softening of acid fragmentals, so the gabbro may probably have 

 been the result of the metamorphism or re-fusion of the Keewatin 

 greenstones. 



The anorthosite masses of the Beaver Bay diabase, supposed by 

 Lawson to be of Archean age and to underlie unconformably the 

 Beaver Bay diabase, are believed to represent segregation phases in 

 the main gabbro flow, and to be the same as anorthosite masses in 

 the gabbro proper to the west. 



The Beaver Bay diabase is believed to represent the upper portion of 

 the great gabbro flow, and to be due to the first and greatest movement 

 of the gabbro toward Lake Superior. The Logan sills belong to this 

 part of the gabbro flow. 



The Manitou division of the Kevveenawan includes the surface 

 flows, sills, and dikes which accompanied and followed the Puckwunge 

 conglomerate. These eruptives, with the elastics associated with them, 

 do not have a thickness in Minnesota of more than 1000 feet. These 

 lava sheets extend along the shore of Lake Superior from near 

 Baptism River to near Grand Marais, except where replaced at inter- 

 vals by the Beaver Bay diabase or some of the intersheeted fragmentals. 

 They occur also in the neighborhood of Grand Portage Bay, but their 

 extent here is not definitely known. 



General. — The most important petrological conclusions determined 

 from the examination of the Minnesota crystalline rocks, are three in 

 number : 



1. All the granites of the Archean can be explained on the assump- 

 tion that they are intrusives representing the metamorphosed conditions 

 of clastic rocks adjacent to the observed intrusions, rendered plastic 

 by the force of dynamic metamorphism accompanied by moisture. 



