490 GEOLOGY OF CENTRAL WISCONSIN. 



out very readily into rectangular blocks, the planes of easiest cleavage lying at right 

 angles to the bedding. The lowest layer, 3 feet thick, is again of a coarse (949) variety 

 like that of the uppennost bed. 



On the north side of the river, beginning from the point B, we find a continuous low 

 exposure, extending several rods up the stream, and showing the same bedding structure 

 as seen on the south side of the river. Passing over these exposures on the line B C, 

 at right angles to the strike, we find first, for 40 feet, mottled, grayish syenite (951), 

 resembling that on the south side of the river, but somewhat finer in grain. The 

 weathered surface "is of a uniform green hue, and a cross-fracture shows a greenish 

 chloritic crust extending inward as much as j^ inch, with a sharply defined inner edge. 

 Beyond on the line B. C. is first seen a similar rock, which is however, more highly 

 quartzose and felspathic, the amphibole being almost excluded (952). Beyond again green 

 chlorite begins to appeal-, gradually increasing in quantity, the former dip and strike 

 joints at the same time becoming confused by the introduction of schistose planes, until, 

 at 80 feet from the beginning, the rock has become a well-marked green chloritic schist, 

 the schistose planes bearing N. 45 E., and dipping 60 to 80 S. E. The gradation of 

 the one rock into the other is unmistakable. Beyond again the chlorite-schist is largely 

 exposed, and extends entirely across the river, forming the barrier rock of the falls. Its 

 most common variety (953) is dark green in color, with large, interlocking, greasy-sur- 

 faced laminae. The schistose surfaces are readily scratched with a knife, but much sili- 

 cious matter is present. Pyrite is also to be seen throughout. In places the im- 

 purity is less than usual, and the rock nearly all chloritic (954). After crossing about 

 25 feet of this schist, its lamination lines are seen to become again obscure, signs of the 

 former low dip reappearing, and the rock becoming again l.ke that below (955). 



We seem to have in these rocks an instance of the change of an amphibolic to a 

 chloritic rock, with a simultaneous production of schistose planes crossing the ordinary 

 bedding lines. 



At Marathon City, Sec. 6, T. 28, R. 6 E., a low exposure occurs on the edge of the 

 water in Rib river, which shows syenite (957) closely allied to the coarser syenite at Rib 

 river falls. It is medium-grained, dark-greenish and grayish, showing surfaces of bril- 

 liant black lamellar hornblende up to ^ inch in diameter, embedded in a matrix of very 

 fine-granular quartz, and coarser, white, glassy felspar. The hornblende facets fre- 

 quently show a tendency to alteration, and are then ill-defined on the edges, from the 

 surrounding matrix. 



The rocks of Rib river falls and Marathon City bear a resemblance to those found 

 crossing the Wisconsin at Mosinee, but are rather more chloritic, or altered. The strike 

 directions at the two places, N. 5 e to 10" W. at Mosinee, and N. 8 W. at Rib river 

 falls, also correspond. It seems probable that the two are portions of a continuous belt 

 trending west of north. If so, the belt must have a considerable width, for the strike 

 direction at Mosinee, if carried out northward, would not reach so far west as the falls 

 of Rib river. 



YELLOW RIVER VALLEY. 



The upper part of Yellow river, in Wood county, north of the Green Bay and Minne- 

 sota Railroad, flows over gneissic and granitic rocks, which are exposed nearly continu- 

 ously in the beds and on the sides of the stream, for many miles. The same is true of 

 the branches of the river in this part of its course. 



On the divide between Yellow and Black rivers, sandstone extends far to the north- 

 ward, covering, and for the most part concealing, the crystalline rocks, which, however, 

 occasionally rise through the sandstone. The boundary between the formations it is 

 almost impossible to trace accurately, since the irregular surface of the crystalline rocks 



