ALGONKIAN ROOKS OF STUEGEON EIVER TONGUE. 471 



areas seem also to be dififerent in origin, although this can not be stated 

 with certainty, since the origin of the Marquette schists is not so clear as is 

 that of the Sturgeon River schists. There is enough similarity between the 

 crystalline series in the two areas to leave no doubt as to their practical 

 identity. If the Marquette Basement Complex is Archean, the crystalline 

 series underlying the conglomerates in the Sturgeon River tongue is also 

 Archean. 



THE ALGONKIA^ST TROUGH. 



The sedimentary rocks comprised within the Stui'geon River Algonkian 

 tongue may be separated into a conglomerate series and a dolomite series. 

 The conglomerate series consists of schistose conglomerates, arkoses, quartz- 

 ites, slates, and certain sericitic schists that are squeezed arkoses. The 

 dolomite sei'ies embraces crystalline dolomites, a few thin beds of quartz- 

 ite, a few breccias and conglomerates, and some slates. 



It is possible that a third series, composed essentially of slates, also 

 exists in the district, but if so it is not advisable to separate it from the 

 dolomite series, since its exposures are very few in number, and the slates 

 which comprise its main mass are so nearly like the slates belonging in the 

 dolomite series that they can with difficulty be distinguished from, these. 



Associated with the sedimentary rocks are great masses of .basic 

 igneous ones. Some of these are unquestionably intrusive masses, as 

 shown by their relations to the conglomerates, while others appear to be 

 interleaved sheets. A very few, apparently bedded greenstones, on close 

 examination seem to be composed of intermingled sedimentary and igneous 

 material. These may be altered tuffs. 



Nearly all the sedimentary as well as the igneous rocks embraced in 

 the trough are schistose, and thus are sharply distinguished from the brown 

 Potsdam sandstones and the Silurian limestones that here and there He 

 approximately horizontal on their upturned edges. The squeezing of the 

 pre-Potsdam formations has been so intense that both conglomerates and 

 dolomites have been forced into closely appressed folds, which in the con- 

 glomerates are for the most ^jart apparently isoclinal. The strike of the 

 latter rocks is nearly east and west, and their dip nearly perpendicular, 

 except in one or two cases. The dolomites are less closely folded than the 

 conglomerates. Their dips are much less steep, and their strike varies 



