HYDROLOGY. 133 



line of an apparent fault, the limestone is brought athwart its course, 

 and it turns to the west, still following the face of the Lower Magne- 

 sian cliffs, until they turn southward in the town of Mukwa, when the 

 river curves in the same direction, and at length in the bed of Poy- 

 gan lake and the basin of the Fox river, it finds its way across the 

 obtrusive formation. Its waters then reverse their course and flow 

 back along the face of the projecting cliffs of the Xiagara limestone 

 for a hundred miles, when Porte des Morts allows them to escape into 

 the great lake, at a point not half the distance from their source that 

 they have traveled. 



The Oconto river, on the contrary, on encountering the resisting 

 Lower Magnesian limestone, turns sharply to the east and flows along 

 the north face of the formation for some distance, when it forces its 

 way across it, forming the beautiful falls of the Oconto, and keeps 

 directly on its course to the bay. The falls are occasioned by a soft 

 shaly stratum near the middle of the formation, there being hard, 

 heavy bedded layers of dolomite above and below. The softer stratum, 

 being more easily eroded, permits the water to undermine and throw 

 down the heavy beds above it, thus keeping the face of the cliff verti- 

 cal and causing the falls to slowly recede. The perpendicular fall is 

 about twenty-two feet, with a considerable descent upon the rapids 

 above and below. 



The deflection of the Menomonee river to the eastward (town 33, 

 ranges 21 and 22 E.) is to be attributed to the barrier interposed by 

 the same formation, though in this instance it is far less conspicuous, 

 as the formation does not immediately adjoin the river on the south, 

 nor does it anywhere in the vicinity project in mural cliffs, according 

 to its habit, to the southward. Yet its influence on the drift accumu- 

 lations is apparent, and it is none the less the cause of this deviation 

 of the river from its general course. At Grand Rapids the river 

 crosses the formation, the rapids being due to the same cause as the 

 falls of the Oconto. The shaly stratum is here harder, however, and 

 the layers above less massive, making the resisting power of the two 

 portions less different, so that the result is a series of rapids instead 

 of vertical falls. Immediately on passing this barrier, the beautiful 

 river recurves to the south, indeed to the west of south, and follows 

 the horizon of the St. Peters sandstone, having the Trenton limestone 

 on the east, until within about eight miles of its mouth, where it 

 crosses the latter formation by a succession of rapids. It is true that 

 between the Grand Rapids and those last mentioned, neither the St. 

 Peters sandstone nor Trenton limestone appear as obvious barriers, 

 the channel of the stream being excavated in drift, but there is good 



