THE LOWER SILURIAIT ROCKS.' 533 



western corner of Juneau county. This line is, however, anything 

 but a regular one, the Lower Magnesian occurring in more or les* 

 detached areas crowning the summits of the higher grounds. In 

 Juneau county only a few small summits in the southwest corner 

 reach the Lower Magnesian horizon, the rest of the county being 

 well down in the Potsdam series. West of Juneau county the 

 boundary is without the Central Wisconsin district. 



The topographical characters of the regions in which the Pots- 

 dam is the surface rock have already been generally given in the 

 chapter on Surface Features, and further details are given in the 

 latter half of the present chapter. It may be said, in general, that 

 where this formation is at surface there is usually a loose, sandy, 

 sterile soil, a sparse growth of small oak timber, mingling with and 

 becoming replaced by small pine towards the north, and a general 

 plain-like character, the plain dotted with lofty and rocky outliers 

 of the same formation, or of the next higher one. To these general 

 statements there are exceptions, the principal of which .may be here 

 given. Excellent soil is found within the Potsdam area where the 

 drift covering is heavy, as in parts of Waushara county, or where 

 alluvial depositions exist in the valley bottoms, as in places along 

 the Wisconsin valley, or yet again where a rough, ridgy charac- 

 ter to the country prevails, as in southwestern Juneau county, where 

 good land occurs on the top of the ridges, being due possibly to the 

 tendency of the loosened sand to wash downwards towards the valleys. 

 To the general plain-like character of the Lower sandstone area, south- 

 western Juneau county and northwestern Sauk make an exception, 

 the Baraboo river and its tributaries having worn the ridge that bounds 

 the central sand plain into an adruptly ridgy country. Another ex- 

 ception is found in parts of Waushara county, where morainic drift 

 occurs in great abundance. 



The general lithological characters and stratigraphical ar- 

 rangement of the Lower sandstone series will be best understood 

 from a brief summary of the main facts obtained in the different dis- 

 tricts in which the formation is at surface. 



At Madison, Dane county, the Artesian well in the Capitol park 

 passes through 126 feet of loose materials, apparently all belonging 

 to the Drift, 704 feet of sandstone, for the most part purely silic- 

 ious, light-colored, and fine-grained, the constituent gi'ains being 

 all more or less rolled, and cemented by a varying, but always 

 very small, amount of hydrous iron oxide — and 10 feet of a red shale, 

 imderneath which are the crystalline rocks. Similar results are ob- 

 tained from the well at the Milwaukee and St. Paul depot at Madi- 



