OUTLINE OF MONOGRAPH. 23 



places favorable for its accumulation, such as the bottom of this synclinal trough, 

 where it was precipitated by oxygen-bearing waters coming more directly from the 

 surface. Pari passu with this precipitation silica was removed, affording space for 

 the accumulation of the iron to form the ore dejoosits as now known. The Tower 

 and Soudan deposits differ only in detail from the Ety deposit. They were accu- 

 mulated in favorable places both at the bottom of the formation, where it rests 

 against the greenstone in which it is infolded, and within the formation in basins 

 formed by the intrusion and subsequent folding of igneous rocks. The mode of 

 accumulation in these is the same as that brieflj' outlined foi- the Ely deposits. 



The methods of mining in the Vermilion district are briefly described. 



In Section IV are described certain acid intrusives var3'ing from line- to coarse- 

 grained granites, and from porphja'ies with vevy fine-grained grouudmass to granite- 

 porphyries. The granites are known from the topographic features with which 

 they are associated, as the granites of Vermilion, Trout, Burntside, and Basswood 

 lakes, the granite between Moose Lake and the Kawishiwi River, and the granite of 

 Saganaga Lake. All of these rocks are j'ounger than the Ely greenstone, for they 

 occur in it as dikes. A number of these dikes are found also in the iron-bearing 

 Soudan formation, which is of more recent origin than the greater part of the Ely 

 greenstone. That these intrusives are older than the Ogishke conglomerate (Lower 

 Huronian), which succeeds in age the Soudan formation, is shown conclusively' by 

 the fact that pebbles derived from them occur in this conglomerate. The general 

 period of intrusion of all of these acid igneous rocks is placed between the time of 

 the deposition of the latest sediments of the Archean and that of the deposition of 

 the earliest sediments of the Lower Huronian series. Some were perhaps intruded 

 near the beginning of this interval, others probably near the end, but it is now 

 impossible to give their exact ages. In the portion devoted to the granites of the 

 different areas the various intrusives are described somewhat in detail. Their petro- 

 graphic characters are given as hornblende- and mica-granites, and the various 

 schistose rocks produced from them are described. 



Chapter IV. This chapter is devoted to a description of the Lower Huronian 

 series. In Section I are discussed the sedimentary rocks of this series, which have 

 a very large surface extent in the Vermilion district. They are present in two 

 large detached areas, one of which, known as the Vermilion Lake area, extends from 

 the western limit of the area mapped, in the vicinit}' of Tower, to within about 11 

 miles of Ely on the east. The second area begins about 7 miles west of Ely and 

 extends eastward to the eastern limit of the area mapped. This is known as the 

 Knife Lake area. The rocks of these two areas, although of slightlj^ different 

 petrographic character, are of essentially the same age. At the base of the series 

 there lies a great conglomerate, known as the Ogishke conglomerate. The relation 

 of this conglomerate to the formations previously described is conclusively shown 

 by the fact that it consists of pebbles and finer detritus derived from the Ely green- 

 stone, the Soudan formation, and the various acid intrusives already mentioned. 

 Above this conglomerate in the eastern portion of the district there are found in a 



