THE ARCHAEAN ROCKS. 



485 



feet in diameter. On the summit are to be seen some large irregular exposures of the 

 same rock (924), showing no sign of bedding structure. None of the quartzite of either 

 hill has any trace of lamination. 



About three-quarters of a mile immediately cast of the Upper hill, at the water's 

 edge on the west bank of the Wisconsin, is a low outcrop, several hundred feet in 

 length, of a reddish syenite. At the upper end of the exposure the rock (919) is very 

 coarse, composed of a deep-red, cleavable felspar, mottled with patches of brilliant 

 black hornblende, up to a quarter of an inch in diameter, and showing a small quantity 

 of translucent, brownish-stained quartz. The deep red color is evidently partly due to 

 weathering. Two sets of widely separated joints occur, one set, the most marked, 

 bearing N. 30 E., the other N. 10 W. A hundred feet below, this rock changes to a, 

 very light-colored, fine-grained variety (920), poor in hornblende; and immediately be- 

 low again to a very coarse kind (921) bluish-grey in color, owing to the preponderance 

 of large surfaces of bluish, cleavable, non-striated felspar, and mottled with black 

 patches of hornblende. This is evidently the normal variety from which the reddish 

 crumbling kinds result by weathering. The rock of this exposure is the same that is 



FIG. 15. 



QUARTZITE EXPOSURE ON RIB HILL, MARATHON COUNTY. 



largely displayed at Big Bull falls, five miles to the north, and is entirely unlike any 

 rock noticed farther down the stream. 



Rib hill, on Sees. 8 and 9, T. 28, R. 7 E.', shows large exposures of the same sort 

 of quartzite as that occurring on the Mosinee hills, three miles southeast. This hill is 

 a bold isolated crest, about a mile in length, trending north of west, across the southern 

 half of Sec. 8, and gradually increasing in height from an altitude of 1,143 feet at its 

 eastern extremity, on the western side of Sec. 9, to one of 1,263 feet at its western ex- 

 tremity near the west line of Sec. 8. This western end is thus, so far as definitely 

 known, the highest land in the state. It rises 660 feet above the Wisconsin river, three 

 miles east, and 620 above the railroad track at Wausau. The summit of the hill is 

 rather flat, and is traversed longitudinally by a line of precipitous exposures of quart- 

 zite, from five to forty feet in height. The slopes on all sides are very steep and are 

 covered with a heavy talus of loose, angular masses of quartzite, of all sizes. The 

 northern side is the most abrupt. For several hundred feet it slopes away from the 

 summit at angles of from 25 to 30. 



The exposures and talus show everywhere but the one kind of rock (927), a hard, brit- 

 tle, non-laminated, glassy translucent quartz, usually of a dirty white color, but often 



