24 OUTLINE OF MONOGRAPH. 



few localities small masses of the iron-bearing Agawa formation. ,This formation 

 is petrographically the same as the Soudan formation. In it, however, there is in 

 places a development of the carbonate-bearing facies. No iron ores have been 

 found in it, and it is of so small a surficial extent, and so thin, that no large iron-ore 

 deposits will probablj'^ ever be found in it in the United States in the Vei-milion 

 disti'ict proper. A reconnaissance made in the adjacent portion of Ontario indicates 

 that it is there better developed than on the United States side of the border, and it 

 may possibly contain iron deposits in this area, although this does not seem to be 

 very probable. This iron-bearing formation is wanting in the western portion of 

 the Vermilion district. 



Overlying the Ogishke conglomerate in the western portion of the district and 

 the intervening iron-bearing Agawa formation where present in the eastern portion 

 of the district, there occurs a thick series of slates of varying character, to which 

 the name Knife Lake slates has been given. These slates have been very closely 

 folded. Owing to the lack of well-defined horizons in the conglomerates and in the 

 slates it has been impossible to trace out the structure of this series b}- following 

 key rocks. The folding has, however, been proved in many localities bj' a study of 

 the distribution of these rocks. The relation of this series to the older rocks is 

 shown by the fact that it consists of detritus derived from these older rocks. In 

 three large areas granites which are younger than the sediments are associated 

 with them. These granites are known as the Giants Eange granite, the Snowbank 

 granite, and the Cacaquabic granite. This relationship is proved by the fact that 

 these granites cut through, send offshoots into, and have metamorphosed the 

 sediments. As a result of this metamorphism, micaceous conglomerates in which 

 the conglomeratic structure is still recognizable have been produced from the 

 Ogishke conglomerate, and mica-schists have been produced from the Knife Lake 

 slates. These sediments are also metamorphosed by the Duluth gabbro, which has 

 changed them into mica-schists. Hence the gabbro is younger than the sediments. 

 In addition there are found in the rocks of the series certain basic dikes which are 

 similar to others which cut the Duluth gabbro, and which are considered to be of 

 Keweenawan age. 



In Section II of this chapter various acid intrusives of the same general 

 character petrographically, and of the same geologic age, are discussed. They are 

 granites and granite- porphyries which occur in large masses and in dikes penetrating 

 the surrounding Lower Huronian sediments and other adjacent rocks. From their 

 occurrence in the vicinity of the Giants Range, Snowbank Lake, and Cacaquabic 

 Lake these names have been given to the granites occurring in these areas, 

 respectively. There is included also a description of some acid and intermediate 

 intrusives of the same age as the large masses of acid intrusives. The Giants Range 

 granite is a hornblende-mica-granite, and varies from very fine-grained rocks 

 through medium-grained to coarse-grained rocks. The Snowbank granite also 

 varies from fine- to coarse-grained forms, with medium-grained facies as the most 



