INTERESTING LOCALITIES OF ISHPEMING FORMATION. 443 



disappearance of the larger fragments the conglomerates resting upon tlie 

 Archean pass up into like rocks. The d3'namic forces were not severe 

 enough to entirely destroy the large original rounded fragmental grains of 

 quartz. These are represented by roundish or oval areas, which may show 

 only undulatory extinction or fracturing, or may be oblong granulated areas- 

 But the latter differ from the tiner-grained background in the absence of 

 muscovite. However, the quartz grains never show cores with enlarge- 

 ments. They completely interlock. Their fragmental character is therefore 

 indicated mainly by the general shapes of the complex areas. In tlie tiner- 

 grained varieties of the quartz-schist no distinction is to be made out between 

 detrital and nondetrital grains. The rock is a completely crystalline, mus- 

 covitic quartz-schist. As in tlie case of the conglomerates, a greater or less 

 amount of feldspar is present 



By an increase in the amount of muscovite the quartz-schists pass into 

 typical crystalline muscovite-schists. In many of the rocks biotite also 

 is a princifial constituent, and they become muscovite-biotite-schists. By 

 a further replacement of the muscovite by biotite the rocks pass into 

 biotite-schists. In some cases the folia of muscovite are arranged trans- 

 versely to those of biotite, as though one of these minerals had developed 

 as a consequence of one dynamic movement and the other of a later one. 

 In a few instances the amount of feldspar in the background is so great as 

 to be a pnncipal constituent, and the rocks are muscovite-biotite-gneisses. 

 The feldspar includes orthoclase and plagioclase, the major portion of 

 the latter being- microcline. The completely crystalline gneiss occurs in 

 association with an intrusive greenstone. 



A portion of the feldspar in the background of the conglomerates, 

 quartz-schists, and mica-schists appears to be detrital, but a large part is 

 apparently a new development, as much of it is perfectl}' clear, showing no 

 decomposition, and including magnetite and other minerals. In the quartz- 

 schists and mica-schists, magnetite, epidote, and zoisite are often abundant 

 accessories, and in the mica-schists garnet also is plentiful. The mica-schists 

 of the Ishpeming formation are the same in their general characters as those 

 in the Michigamme formation (pp. 447, 449-450), and they will therefore 

 not be here described in detail 



