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as first shown by Spurr, but the ores have come also in small 

 part from the alteration of iron carbonate which is correlative 

 in origin with the green granules. The ferrous silicate granules 

 are believed not to be glauconite, as they were named by Spurr. 

 Comment. — The reference to the Archean of sedimentary iron 

 formation rocks in the Vermilion, Michipicoten, and Marquette 

 districts is a source of surprise and comment among many inter- 

 ested in pre-Cambrian stratigraphy. The term "Archean" has 

 been consistently used by the United States Geological Survey 

 for the " basement complex " of the Lake Superior region, 

 consisting essentially of igneous rocks, and all pre-Cambrian 

 sedimentary rocks have been referred to the Algonkian. In 

 survey reports on Lake Superior geology there have been slight 

 variations from this usage, for certain " tuffs" and "gneisses " 

 (Kitchi and Palmer) classed as Archean in the Marquette dis- 

 trict have been referred to as partly sedimentary. In the present 

 paper Van Hise has gone a step farther and included sedi- 

 mentary formations of considerable importance in the Archean. 

 The basal complex rocks of igneous origin are very closely 

 associated with sedimentaries, and in many areas scarcely to be 

 discriminated in the mapping. On the other hand, both igneous 

 and sedimentary rocks are sharply separated by a profound and 

 conspicuous unconformity from sediments called Huronian or 

 Algonkian. In mapping in the Lake Superior region it has been 

 found convenient, and in many cases necessary, to class together 

 all rocks, igneous and subordinately sedimentary, below this well 

 recognized unconformity at the base of the Huronian. Van Hise 

 has chosen to retain the term "Archean" for this structural unit. 

 It is thought by many geologists that it would have been prefer- 

 able to have extended the term " Algonkian " to include the sedi- 

 ments beneath the well recognized Huronian, thus adding one 

 more series to the Algonkian, keeping in the Algonkian all recog- 

 nizable sedimentary rocks beneath the Cambrian, and leaving 

 the term Archean for the true igneous basement complex (if there 

 be such a thing). If, because of the close association of the 

 basement igneous rocks and the sedimentary rocks beneath the 

 Huronian, it were found desirable to consider the basal igneous 



