430 THE CRYSTAL FALLS lEON-BEAEmi:^ DISTRICT. 



biotite plates of medium size. Minute grains of blue quartz are occasionally 

 distinguishable by the eye. 



Under the microscope these gneisses have a fine to medium grained 

 groundmass composed of quartz, microcline, orthoclase, plagioclase, green 

 biotite, and muscovite, and a little scattered epidote. Within it are large 

 roundish areas of quartz and feldspar, sometimes single individuals, but more 

 often consisting of several fragments. The gneissic foliation is pronounced 

 and is caused by a general elongation of the constituent minerals in a 

 common direction. The only essential differences between these gneisses 

 and the porphyries described above are this strong foliation and the coarser 

 groundmass. 



SECTION II. THE STURGEON FORMATION. 



The Sturgeon formation as a distinct member of the Algonkian series 

 is hardly known in this area apart from the Randville formation. Neverthe- 

 less, purely clastic sediments unmixed with the carbonates of calcium and 

 magnesium were deposited and are now visible along one section between 

 the Archean granites below and the dolomites above, and for these it is 

 convenient to retain the name, although their total thickness is so small 

 and their continuity so uncertain that they can not be shown on the geo- 

 logical map. The general conditions of sedimentation here were such, 

 perhaps in consequence of the low relief of the neighboring land, that lime- 

 stones began to form a relatively short time after the submergence of the 

 Archean surface, so that the two lower Algonkian formations probably by no 

 means represent equal periods of time with the same formations in the Felch 

 Mountain range. The time represented by both together is perhaps not 

 greatly difi^erent in the two areas, but since in the entire absence of fossil 

 evidence it is impossible to draw the line of equivalence, while at the same 

 time the lithological break is a sharp one, it seems desirable to carry over 

 the Felch Mountain names, extending the Randville dolomite downward to 

 the lower limit of hmestone deposition, and retaining the name Sturgeon 

 formation for the basal sediments which are free from carbonates. 



These basal sediments are found only in sec. 15, T. 44 N., R. 31 W., 

 where they are exposed in low-lying outcrops in the banks and bed of the 

 Fence River. Elsewhere throughout the 10 or 12 miles through which the 

 Ai-chean extends in this area no outcrops have been found in the flat and 

 generally swampy belt which intervenes between it and the dolomite above. 



