436 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



The ratio of CaCOg: MgCOg is too great for normal dolomite, but 

 approximates that for 2CaC03+MgC03. 



Under the microscope the chief differences in the various thin sections 

 are in the degree of metamorphism and in the quantity and character of the 

 foreign fragments. The least altered A^arieties are those highest in the series 

 from the Michigamme Mountain area. These consist of a background of 

 extremely fine-grained calcite, with a few rounded fragmental quartz grains, 

 and scattered particles of chalcedony. Mixtures of small quartz particles, 

 chalcedony, and calcite slightly coarser than the background occur in short 

 vein-like gashes. The prevalent deep color of these rocks is due to the even 

 sprinkling through the background of a black opaque pigment, which may 

 be carbonaceous. Altogether the microscopic characters are those of a 

 little-altered, slightly cherty limestone. 



The more crystalline varieties of the dolomite contain several secondary 

 minerals, namely, tremolite, diopside, chlorite, muscovite, phlogopite, pyrite, 

 and the oxides of iron. Of these, tremolite is very common and abundant, 

 especially in the Fence River area, where the rarer pyroxene, diopside, also 

 is found. Phlogopite comes in but two of the thin sections, while muscovite 

 occurs in nearly all. The general habit of these silicates is precisely the 

 same as in the dolomite of the Felch Mountain range. They are developed 

 pari passu with the passage of the unaltered dolomite into marble. 



The fragmental inclusions within the dolomite are of interest. These 

 are little pebbles of quartz, feldspar, mica, titanite, magnetite, and augite; 

 and are evidently dei'ived mainly from preexisting granites or gneisses. 

 Titanite and augite are very rare; the others are represented in almost 

 every slide. The quartz grains are seldom more than a millimeter in diam- 

 eter and commonly are much smaller. While the general shape is oval or 

 rounded in most cases, the perimeters are usually extremely irregular and 

 interlock with the carbonate grains of the background, which indicates that 

 they have been enlarged since deposition by the formation of new silica. 

 This is very evident in the few instances in which the original smooth out- 

 line, or part of it, is preserved by a film of different material inside the 

 present perimeter. The feldspar pebbles include orthoclase, microcline, and 

 plagioclase, microcline being the common species. They are usually much 

 decomposed and iron stained. The feldspars are especially abimdant in the 

 slides from the Fence River area. 



