170 THE CUYSTAL FALLS IRON-BEARING DISTRICT. 



(if these niiuerals occur tog-ether, but more commonly one finds various 

 combinations of certain of them. When muscovite is present in large 

 (juantity, it is usually not accompanied by biotite or chlorite, the iron and 

 magnesium necessary for the production of biotite and chlorite evidently 

 not having been present. These last two, however, are always associated. 

 As the mica increases, the schistosity of the rock increases in a, corre- 

 sjionding' manner, and the rocks become tli<ise which may be spoken of as 

 micaceous graywaekes. 



These micaceous graywaekes represent a somewhat more advanced 

 stage of metamorphism of the rocks than the graywaekes just described, 

 and the extremely altered varieties of these are very close to the mica- 

 schists and inica-gneisses, according to the respective amounts of secondary 

 feldspar present. No distinction, however, can be made in the field between 

 some of the less metamorphosed graywaekes and these micaceous ones. 

 The chief difference appears to be in the fact that in the micaceous gray- 

 waekes the larger feldspars are almost completely altered and the finer 

 matrix completely recrystallized into readily distinguishable mineral parti- 

 cles. In these more metamorphosed rocks the parallel intergrowth of 

 secondary muscovite and biotite is nicely shown, a thin leaf of biotite being- 

 included between two lamellae of muscovite. A considerable quantity of 

 epidote is scattered in large grains through the micaceous graywaekes, 

 besides occurring in aggregates of small grains. Some crystals of apatite 

 and tourmaline were observed. Rutile is found in some quantity, and with 

 it is also sphene, both of them possibly resulting from the alteration of 

 titanium-bearing iron ores in the original graywaekes. The iron present 

 in the original graywaekes as siderite and the minute specks of oxide have 

 been collected into large crystals of magnetite and als(i into aggregates of 

 smaller, well-defined magnetite crystals. 



The alteration of the feldspar and the production from it of quartz, 

 secondary feldspar, epidote, and mica is well shown in one case. In this 

 the nucleus of original feldspar, in the center, contains minute grains of 

 epidote and flakes of muscovite, besides reddish, presumably ferruginous, 

 specks. These with a low power cause the feldspar to appear cloudy. 

 Surrounding- this core is a mottled zone in which secondary feldspar and 

 quartz occur. In one place in this zone a flake of biotite is observed. Some 

 ej)idote also occurs in it, but no muscovite. 



