INTERESTING LOCALITIES OF JMIOHIGA.MME FOJJMATION. 453 



grained and more carbonaceous rocks stain t\w iinj^crs, and are so soft as 

 to readily give a black mark. When parted along the cleavage, many of 

 them show lustrous graphite. In all of the carl)onaceous varieties of rock 

 pyrite and marcasite are verv plentiful Even the fine-grained gravwackes 

 have a dark color, due to the contained carbon. For the most part the 

 rocks show comparatively feeble dynamic effects. To the majority of 

 the rocks the term sliales and grits Avould almost be ajjplicable. Slaty 

 cleavage is present only in the iiue-grained varieties. As has been explained, 

 occasionally the movements have been sufficient to develop lustrous graphite 

 between the lamina? of the slates. In places, as a result of the movements, 

 the slates were broken, and the cracks filled with vein quartz, the veins 

 varying from minute seams to those several inches across. 



In thin section the rocks correspond in their characters, in most respects, 

 to the general description of them given on pages 448, 450-451. The coarser- 

 grained gray wackes contain comparatively little feldspar and a small amount 

 of interstitial material. They are largely cemented by enlargement. They 

 therefore approach (piartzites. They are, however, always discrhninated 

 from the quartzites of the Ishpeming formation by the presence of lilack 

 carbonaceous material between the grains. The dynamic effects observable 

 under the microscope are usually slight, the grains only occasionally being 

 ai'ranged with their longer axes in a common direction, although they 

 commonly show undulatory extinction and fracturing, sometimes in a rectan- 

 gular manner. In the few slides in which the quartz grains have a parallel 

 aiTangement sericite is abundant, this appearing to be developed in propor- 

 tion to the other dynamic effects. In both the slates and graywackes mica 

 and quartz have developed by the decomposition of the feldspar in the 

 interstices. Biotite is the predominant mica, although chlorite is rather 

 plentiful. This process is of far greater importance in the slates than in the 

 quartzites. The carbonaceous material is so abundant in many of the black 

 slates as to make it difficult to determine the minerals present, and especially 

 the amount of iron oxides. In a few of the slates the movements have 

 been sufficient to develop folia of biotite in a parallel direction, and thus 

 make them mica-slates. In the transverse sections of the graphitic slates 

 the contorted laminae of graphite are beautifully shown. 



