448 TUE MARQUETTE IKONBEAIUNG DISTKICT. 



schist. It is concluded that the pegniatization was not the result of an 

 igneous injection from an extraneous source, particularly as there ai-e no 

 known granite intrusives within the Upper Marquette series. The facts 

 seem rather to be explained by water action. The whole rock must have 

 been permeated by hot solutions, from which the new minerals separated 

 in the interspaces left by the folding. 



If this explanation be correct, the rock is one to which the term "meta- 

 morphism" is applicable. Why the rocks of this part of the formation are 

 so thoroughly metamorphosed and those to the east comparatively little 

 affected has not been certaiidy determined. The beds are intensely plicated. 

 Such plication involves a large amount of readjustment of the layers over 

 one another and within the layers themselves — that is, the mashing was 

 exceedingly severe. During this time of folding, by the decomposition of 

 fragmental feldspar into quartz and mica, the development of new feldspars 

 in some places, and the granulation of the coarser crystalline quartz, the 

 rock changed into a mica-schist or a mica-gneiss. 



Microscopical. — Thc slcitcs and grayivaches consist mainly of fragmental 

 quartz and feldspar set in a clayey and micaceous matrix. Occasionally 

 other fragmental constituents, and especially mica, are found. In the cases 

 of the finer- grained slates the clayey matrix is predominant. In the 

 coarser-grained graywackes the plainly fragmental material is predominant. 

 In the latter we often have closelv fitting grains of quartz, some of them 

 well rounded and enlarged, with a, few of feldspar, set in a sparse matrix. 

 This rock approaches a quartzite. The fragmental constituents generally 

 show pressure effects, the larger grains being broken into two or more 

 fragments, or cut by fine cracks, sometimes in a rectangular manner, 

 which cause undulatory extinction. The fragmental grahas of feldspar 

 have largely decomposed, and quartz, biotite, and chlorite have developed 

 from them. In the clayey background there have developed many minute 

 flakes of biotite, sericite, leaflets of chlorite, and sometimes needles of 

 actinolite. These usually do not have a parallel arrangement. More fre- 

 quently than not there is also present a greater or less quantity of ferrite. 

 Sometimes crystals of tourmaline also occur. 



