OUTLINE OF MONOGRAPH. 21 



chapter is devoted to abstracts of articles dealing with the geology of the district. 

 In these abstracts the authors have been quoted very freely. From a perusal of this 

 chapter one can obtain an idea of the growth of knowledge of the geology of the 

 district, which is comparatively difficult of access. 



Chapter III. This chapter deals with the Archean. 



Section I gives the definition and subdivisions of the Archaen. As a result of 

 studies made largely in this district it was found necessary to modify the definition 

 of the Archean so as to include within it some small quantities of sediments. The 

 Archean of the Vermilion district is divided into three formations, as follows, given 

 from the base up: The Ely greenstone, the iron-bearing Soudan formation, and the 

 granites of Vermilion, Trout, Burntside, Basswood, and Saganaga lakes. 



In Section II the EI3' greenstone is described. This formation consists of basic 

 to intermediate igneous rocks, and is the lowest member of the geologic column. 

 These greenstones are very widely distributed and occur normally in anticlinal areas, 

 as is shown by the distribution of the overlying sedimentaries. A petrographic 

 study of the greenstones shows that they were originally rocks corresponding in 

 character to intermediate andesites and basic basalts. They have been extremely 

 altered, but retain in many cases in striking perfection the original structures, such 

 as ellipsoidal parting and spherulitic and amygdaloidal structures. A studj^ of their 

 various textures and structures shows that these greenstones are unquestionably of 

 igneous origin, and are largely of volcanic character. With the volcanics there are 

 associated, of course, some intrusives of essentiallj^ the same age. These have been 

 ■subjected not only to the ordinary processes of alteration that have metamorphosed 

 the greenstones, but have been strongly compressed and in many cases have become 

 schistose. Actual green schists, however, are very subordinate in quantity. The 

 greenstones have also been strongly affected by the contact metamorphism due to 

 the intrusion of great granite masses. As a result of this intrusion there have been 

 produced from the greenstones amphibole-schists, which form a marginal facies of 

 the greenstones, lying between them and the adjacent granites. The greenstones 

 have also been metamorphosed by the Duluth gabbro of Keweenawan age, and 

 granular rocks have thus been produced which in most cases show the original 

 textures of the greenstones, but contain also a development of fresh biotite, hyper- 

 sthene, brown-green hornblende, and magnetite. These greenstones have very 

 slight value at present, although they make good road material. 



In Section III the iron-bearing Soudan formation of the Archean is ti'eated. The 

 iron formation is widely distributed in the western part of the district, but is 

 practically wanting in the eastern half. Where it occurs it is found mostlj' in 

 narrow belts, which consist largely of greenstone so intimatelj" associated with the 

 iron formation that it has been impossible to separate them on the map. In spite 

 of the resistant character of the rocks constituting the formation, exposures are not 

 veiy good, and it has been difficult to trace out continuous belts. The Soudan 

 being the oldest sedimentary formation in the district has been subjected to all the 



