422 GEOLOGY OF CENTRAL WISCONSIN. 



a southerly course through Clark county as far as township 24, where it 

 begins a southwesterly stretch towards the Mississippi, which it reaches 

 in town 17, range 8, on the boundary line between Trempealeau and 

 La Crosse counties. Its total length is about 150 miles, and total fall 

 about 750 feet. Like the Wisconsin, it has its upper or Archaean por- 

 tion, broken constantly by chutes and rapids over gneiss and granite, 

 and its lower or sandstone section without falls. The Archaean sec- 

 tion of the river extends to the town of Black River Falls, in town- 

 ship 21, Jackson county, where the gneissoid granite and gneiss 

 cause a long rapid and disappear finally beneath the sandstones, 

 which, however, extend for many miles northward of this point on the 

 immediate banks of the stream, covering the crystalline rocks every- 

 where except in the river bed. Towards its mouth the valley of 

 Black river is bounded by limestone-capped bluffs like the lower por- 

 tion of the valley of the Wisconsin. Like the Wisconsin, again, it 

 has its upper waters in a pine covered region, is much used for log- 

 ging, and affords numberless water powers by its rapid descent and 

 frequent rock interruptions. Most of the branches of the upper Black 

 run over crystalline rocks like the main stream, and have numerous 

 rapids and falls. Some of them, however, as for instance the East 

 Fork, reverse the ordinary conditions of the streams of the region, 

 and have their upper portions in sandstone on the high divide near the 

 Meridian in northeastern Jackson and eastern Clark county, whilst 

 farther down they cut into the crystalline rocks, making the usual 

 rapids and falls. 



The main Rock river only touches the southeastern corner of our 

 district in the expansion known as Lake Koshkonong, but its branches 

 drain nearly all of Dane and most of eastern Columbia. These 

 branches are everywhere divided from the tributaries of the Wiscon- 

 sin by the high belt of limestone country already described as run- 

 ning southwestward through eastern Columbia, and then westward 

 through northern Dane. In Dane county are three branches of 

 Rock river, draining three distinct north and south basins. Central 

 Dane is traversed by the Catfish, whose upper waters expand into 

 several large lakes that lie in a series of N". E. S. W. valleys appar- 

 ently of glacial origin. Between two of these lies Madison, the cap- 

 ital city of the state. In the western towns of Dane are the heads of 

 Sugar river, whilst on the eastern side the drainage is into Kosh- 

 konong creek, except on the extreme northeast, where it is eastward 

 to the Crawfish, as is also the case with the eastern part of Columbia 

 county. 



The Fox river, the fourth of the main rivers of the district, heads 



