486 



GEOLOGY OF CENTRAL WISCONSIN. 



FIG. 16. 



almost colorless. No bedding structure was observed, the rock being apparently even 

 Avithout any one persistent set of cross-joints, though all of the exposures show frac- 

 tures, some quite irregular, and others approaching to plane surfaces. At one point 

 several parallel N. E. joints occur. Fig. 15 is a sketch of one of the large exposures 

 near the western extremity of the hill. 



On the Eau Claire river, at the crossing of the Stevens Point and Wausau road, 

 Sec. 7, T. 28, R. 8 E., there is a fall over coarse, pinkish syenite (926) resembling that 

 on the Wisconsin, near the Mosinee hills, and also the prevailing syenite at Big Bull 

 falls, a short distance northward. 



On the upper Eau Claire, in Sec. 4, T. 29, R. 10 E., arg exposures of a very coarse, 

 rough-textured, felspathic granite, consisting of pink, cleavable felspar; very large- 

 flaked, black mica; and gray quartz. 



At AVatisau, on sections 25, 26, 35 and 36, 

 T. 29, R. 7 E., Marathon county, the Wiscon- 

 sin makes bold rapids known as Big Bull 

 falls. Here the river is divided into a series 

 of channels by a number of small rocky 

 islands, and the exposures are large, the rock 

 on all being of one general kind, i. e., syenite 

 or syenitic granite. This syenite varies in 

 degree of coarseness, but is commonly veiy 

 coarse, the separate minerals being very 

 plainly perceptible to the naked eye. There 

 is no resemblance between it and any other 

 rock observed on the Wisconsin river, except 

 that cf the Eau Claire river, and a small 

 outcrop previously alluded to as near the up- 

 per Mosinee hill, both of these being evi- 

 dently merely continuations of the Wausau 

 rock. From the rock of Little Bull falls it 

 differs, (1) in being commonly much coarser 

 in grain, (2) in having usually a more jagged 

 fracture, (3) in showing greater tendency to 

 iron-stain, and weather, (4) in having the 

 hornblende black instead of greenish-black, 

 (5) in having the felspar orthoclase, and (6) 

 in containing usually some blackish mica. 



The bedding of the rocks at Big Bull is for 

 the most part sufficiently distinct, the general 

 strike being N. 80 E. and the dip from 30 to 

 40" N. W., most commonly about 38. In a 

 few places a marked gneissoid structure, co- 

 inciding with this bedding, and owing to a 

 parallel arrangement of the hornblende, was 

 observed. For the most part, however, the 

 several constituent minerals are quite uniformly intermingled, without arrangement 

 of any kind. In the exposures in the bed of the stream, and on the walls of the various 

 channels, large bedding - plane surfaces frequently show, so that the bedding can usually 

 be made out with very considerable accuracy. The separate layers are often not 

 more than 2, 3, or 4 inches in thickness. 



The sketch map, Fig. 16, serves to show the localities of the various exposures exam- 

 ined at this place. Islands No. 1, 2, 5, 8, 9, 10, 11 and 12 are without bare rock. The 



BIG BULL FALLS 



