GROVELAFD FORMATION IN FELCH MOUNTAIN RANGE. 417 



PETROGRAPHICAL CHARACTERS. 



The rocks of the Groveland formation have a genei'al family hkeness, 

 which makes it very easy to distinguish them in the field from all the other 

 members of the Algonkian series. Among them two main mineralogical 

 kinds may be recognized, the usual one of which consists of quartz and the 

 anhydrous oxides of iron, while the other, which is much rarer, is made 

 up essentially of an iron amphibole, quite similar to the griinerite of the 

 Marquette range, with quartz and the iron oxides as associates. 



As seen in the field, the rocks of the first kind are generally siliceous, 

 heavy, and dark colored, the weight and color, which has a tinge of blue, 

 being due to the presence of abundant crystalline iron oxides. A large part 

 of the silica is easily recognized as crystalline quartz, in some instances, 

 indeed, in the form of detrital grains. The visible iron oxides occur both 

 as little spangles of specular hematite and also in irregular dark-blue 

 masses and single grains, the latter often having the crystalline form of 

 magnetite. Many, if not most, of these last, however, seem to be really 

 martite, as they give a dark purple streak, and in fine powder are not 

 attracted by a hand magnet. 



In the first kind there is much variety in external appearance, deter- 

 mined by the variable proportions in which the chief constituents occur 

 and by the different ways in which these constituents are arranged. Con- 

 siderable areas, for example, consist mainly of granular quartz merely 

 darkened by the intimately mixed iron oxides, and in these, so far as the 

 eye can judge, the rock is a ferruginous quartzite. Closely connected with " 

 such occurrences, or included most irregularly in them, are others in which 

 the ferruginous constituents are so abundant and the quartz so subordinate 

 that they would pass for lean iron ores. Between such rare extremes we 

 find all intermediate proportions of mixtures of the quartz and the iron 

 oxides. 



One form of an-angement of the constituent minerals is in narrow 

 parallel bands, in which the quartz and the iron oxides alternately predom- 

 inate. Such alternations are sometimes so frequent and regular as per- 

 fectly to reproduce the -lean "flag ores" of the Marquette range.^ Regular 

 banding, however, is not common. Usually the light or dark bands are 



' Geol. Survey Michigan, Vol. I, Part I, by T. B. Brooks, pp. 93-94. 

 MON XXXVI '27 



