PETROGEArHIOAL CIIAIIACTEK OF SIAMO SLATE. 317 



isoclinal folds. Usually the slaty cleavage nearly corresponds with the 

 longer limbs of these folds, and cuts across the bedding of the shorter limbs. 

 Nowhere is the slaty cleavage so regular as to furnish roofing slates. At 

 some places when the rocks were in the zone of fracture there ■was so 

 nuu-h movement along the cleavage planes and between the beds as to 

 develop distinct slickensides, the rock ])arting into irregular blocks with 

 sides parallel to the bedding and to the cleavage. Each block was 

 smoothed by movement along two sets of shearing planes. The cleavage 

 therefore passes into a fissility.^ In the most extreme stage of alteration 

 the rock is a crystalline mica-schist, with well-developed mica folia. In 

 proportion as the rocks are coarse-grained, slaty cleavage is not devel- 

 oped in them, and it is entirely absent in the coarser-grained graywackes. 

 In general, the rocks of the formation have yielded to the forces to 

 which they have been subjected by folding and mashing, but occasion- 

 ally the coarser phases are brecciated, and rarely they become reibungs- 

 breccias. This indicates that the formation is more plastic than the 

 other Lower Marquette formations, in wliich autoclastic rocks are very 

 common. 



The normal varieties of the formation are not heavily ferruginous, but 

 at the upper and lower horizons the slates contain a great deal of iron 

 oxide and, localh^ interlaminated layers of chert and ferruginous chert, or 

 even griineritic schist. The contact plane between the Siamo slate and 

 the Ajibik formation seems to have been one of the major planes of differ- 

 ential movement, and thus numerous cracks and crevices have formed, 

 which have been taken advantage of by iron-bearing solutions from above. 

 The concentration of ferruginous masses at this horizon, although occur- 

 ring on a comjjaratively small scale, is analogous to the concentration of 

 the ore bodies on impervious basements in pitching troughs, as explained 

 in Section VI. At the upper horizon the slate changes by gradation or by 

 interlaniination into rocks belonging to tlie Negaunee formation. The 

 ferruginous phases are usually hematitic or magnetitic slates, but occasion- 

 ally interlaminated or intermingled with the slates are layers of chert or 



'Principles of North American pre-Camhrian geology, hy C. R. Van Hise: Sixteenth Ann. 

 Kept. U. S. Geol. Survey, Part I, 1896, pp. 651-656. 



