214 THE MARQUETTE IRON-BEARING- DISTRICT, 



quartz and feldspar in the gneissoid granites. In these rocks the mosaic 

 (which in fresh specimens consists of tiny fragments of quartz, orthoclase, 

 plagioclase, etc., broken from the larger grains and saturated with newly 

 deposited microcline and albite) has been changed to an aggi'egate of tinj- 

 flakes of kaolin, chlorite, and sericite, small grains of quartz, and occa- 

 sionally long laminae of muscovite, besides fragments of clouded feldspar. 

 As the alteration of the mosaic proceeds and its decomposition products 

 increase in quantity, its structure becomes less and less clearly recognizable, 

 until in one or two instances it can hardly be discerned. The large frag- 

 ments of feldspar that are embedded in it have also suffered alteration, and 

 the quartzes have been crushed until their positions are occupied by four or 

 five differently orientated grains, which in the less schistose rocks may be 

 seen to fit together into a single one. In the more highly foliated phases 

 the parts have sometimes been moved from their places and now appear as 

 isolated fragments. 



In the Palmer gneisses all certain traces of their origin have dis- 

 appeared. Under the microscope there is but little variation in the structure 

 or composition of their different phases. Even the schistose fragmental 

 rocks that are associated with the conglomerates are as nearly like the true 

 gneisses in thin section as they are in the ledge. They may contain a 

 greater quantity of quartz than do the latter rocks, and the grains of this 

 mineral mav be a trifle more rounded in outline. It is doubtful whether 

 these rocks Avould have been separated from the genuine gneisses derived 

 from the granite had their relations in the field not been plain. Even with 

 the care that has been used, it is probable that a few rocks of fragmental 

 origin have been included in the area of the Palmer gneisses. 



In the thin sections of the gneisses quartz grains are observed embedded 

 in a fine-grained matrix of a nearly uniform texture and composition. The 

 quartzes are crushed, as they are in the schistose granites. Often they form 

 lenticules of a. quartz mosaic in which each separate grain exhibits the phe- 

 nomenon of undulatory extinction. When not completely shattered the}' 

 are granulated around their edges, and especially at the ends of the lenti- 

 cules, where mosaics of fine grains have been produced. Portions of these 

 mosaics extend out as long tails in the direction of the foliation of the rock, 



