980 THE GEOLOGY OF MINNESOTA. 



[Origin of gabbro. 



same origin for the gabbro of the Keweenawan and all its derivatives, viz., that the 

 basic Keweenawan is derived from the metamorphism and complete ref usion of the 

 Archean greenstones and their attendants. 



The lines of evidence leading to this result are three: 



1. Structural field relations. 



2. Petrographic and petrologic. 



3. General considerations. 



The term gabbro as here employed covers all the varieties into which gabbro 

 has been divided, viz.: orthoclase gabbro, quartz gabbro, hornblende gabbro, hyper- 

 sthene gabbro, olivine gabbro, gabbro diabase, gabbro dioryte; also noryte and its 

 variations, including muscovadyte and its varieties; also peridotyte and pyroxenyte, 

 troctolyte, diabase and diabase porphyryte, and all the basic igneous rocks that have 

 been discovered in the northeastern part of the state, excepting only the Archean 

 greenstones. A rock consisting only of plagioclase and diallage, which are the 

 minerals required to form a normal gabbro, has but rarely been found. From 

 necessity, in the descriptions of Part II the term gabbro has been employed in a 

 broad sense, and sometimes without precise definition. There is no character nor 

 group of characters which can be relied on for persistence to precisely define this 

 term, for the characters mingle confusedly as to structure as well as to mineral 

 composition, if not in the same slide at least in the same visible rock mass, and if 

 not in the same visible rock mass, in the same general terrane or mountain mass. 

 Over a certain area, or little hill, a group of petrographic characters may prevail. 

 In the adjoining hill or on the slopes of the same hill, some of those characters are 

 replaced by other characters, and on the third hill a slightly different change is 

 noticed. On the basis of such variations a diversified nomenclature has arisen. 

 Yet the great rock mass, extending for over a hundred miles in Minnesota, bearing 

 the same taxonomic relations to adjoining rocks, must be considered as one geological 

 entity, having one origin and one history. 



There are some isolated areas of gabbro wholly detached from this mass, 

 occurring in the midst of Archean rocks which may not be of the same date as the 

 gabbro mass here referred to. Such are found at Little Falls, in Morrison county, 

 at Knife lake on the international boundary and near Philbrook, in Morrison county. 

 The diabases that compose the bulk of the effusive Keweenawan are considered as 

 extreme derivatives of this gabbro mass; still, one of those diabases and some of the 

 largest of the sills in the Animikie exhibit several gabbroid characters, and at Duluth 

 the Beaver Bay diabase has uniformly been called gabbro. 



There are at least three ways in which a normal basic igneous rock may have 

 been modified by the conditions of extrusion, and some of the intermediate rocks 



