1 68 REVIEWS 



original rock of the formation is shown to consist largely of minute granules of green 

 ferrous silicate, thus confirming Spurr's conclusion. The material was called "glau- 

 conite " by Spurr, but is here determined to be a hydrous ferrous silicate entirely lack- 

 ing potash, and thus not glauconite. It is named "greenalite" for convenience in 

 discussion. The cherts and iron ores are shown to develop mainly from the alteration 

 of the greenalite granules. The slates are in thin layers interbedded with the other 

 phases of the iron formation. The paint rocks result from the alteration of the slates. 

 The conglomerates and quartzites form a thin layer from a few inches to perhaps 15 

 feet or more in thickness at the base of the formation. They pass upward into fer- 

 ruginous cherts of the iron formation rather abruptly, though usually at the contact the 

 chert and quartzite are interleaved for a few feet. The conglomerate of the iron 

 formation rests upon Pokegama quartzite, indicating a slight erosion interval between 

 the Biwabik and Pokegama formations, although the interval is not shown by dis- 

 cordance in bedding, which is parallel in both. Heretofore the quartzite and con- 

 glomerate in the iron formation have not been discriminated from the rocks of the 

 Pokegama formation. In the eastern portion of the range the iron formation is in 

 contact with the Keweenawan gabbro and granite, and near this contact has suffered 

 profound metamorphism. The characteristic rocks of this area are amphibole- 

 magnetite-cherts. The thickness of the formation may vary from 200 to 2,000 feet. 

 The average may be 1,000 feet. 



The Virginia slate is essentially a soft slate or shale formation, but it contains 

 graywacke phases, near its base a little limestone, and near its contact with the gabbro 

 is metamorphosed into a cordierite-hornfels. The normal slate phases of the formation 

 may be distinguished with difficulty in isolated occurrences from the slate layers in the 

 Biwabik formation. The separation of the two is of importance to the explorer, and 

 hence an attempt is made to determine criteria for their discrimination. The thickness 

 of the Virginia formation cannot be measured within the district, but from analogy 

 with the Penokee-Gogebic district and the extent of the low, fiat-lying area south of 

 the Mesabi range supposed to be occupied by the slate, the formation is believed to 

 have a very considerable thickness. The slate grades, both vertically and laterally, 

 into the Biwabik formation. 



The entire Upper Huronian series is well bedded, conformable in structure 

 (although having a thin conglomerate between the Biwabik and Pokegama forma- 

 tions), and dips in southerly directions at angles varying from 5 ° to 20% and excep- 

 tionally at higher or lower angles. The series is gently cross-folded, and the axes of 

 the cross-folds pitch in southerly directions. Accompanying the folding is consid- 

 erable jointing, especially in the brittle Pokegama and Biwabik formations. Indeed, 

 in these two formations the folding is brought about mainly through relatively minute 

 displacements along joints, while in the Virginia formation the folding has taken place 

 mainly by the actual bending of the strata. 



The thickness of the Upper Huronian series within the limits of the district 

 mapped may average about 1,500 feet ; but if the total thickness of the slate formation 

 outside the limits of the district be taken into account, the total thickness of the Upper 

 Huronian series is probably several times this figure. 



The relations of the Upper Huronian series to the subjacent formations are those 

 of unconformity, as evidenced by basal conglomerates, discordance in dip, difference 

 in amount of deformation and metamorphism, distribution of the series, and relations 

 to intrusives. 



