44,8 GEOLOGY OF 'CENTRAL WISCONSIN. 



stones, in nearly horizontal beds. These include, in ascending or- 

 der, the Potsdam, or Lower sandstone, the Mendota limestone, the 

 Madison sandstone, the Lower Magnesian limestone, the St. Peters 

 sandstone, the Trenton limestone, and the Galena limestone. The 

 last named covers bnt a very small portion of onr district. The 

 Archaean rocks lie in a great central mass in the northern part of the 

 state, the Silurian formations encircling this mass in imbricating or- 

 der, and, coming to the surface in a series of receding concentric 

 bands, together forming the southern, eastern and western parts of 

 the state. Tlie surface features of the Archaean and Silurian regions 

 differ much. In the Archtean area we have unquestionable proof, in 

 the crumpled condition of the rocks, of its former mountainous char- 

 acter. Denudation has gradually, in the great lapse of time during 

 which these beds have been exposed, stripped it entirely of these char- 

 acteristics, leaving it now merely a high, and, in general, a gently undu- 

 lating area, broken only very occasionally by small isolated ridges or 

 peaks of quartzose rocks, which have remained standing by virtue 

 of their superior hardness and power of resisting chemical action. In 

 minor detail, however, the Archaean area is much roughened by low 

 ridges whose sides frequently show outcropping ledges of tilted rock. 

 In the Silurian area, on the other hand, the original condition must 

 have been a nearly level one, and denudation has worked here in an op- 

 jDosite direction, removing great portions of the horizontal beds, goug- 

 ing them everywhere into valleys, and producing thus narrow ridges 

 and bold isolated bluffs. The horizontality of the Silurian strata, which 

 is more nearly exact in central Wisconsin thaji elsewhere in the state, 

 together with their alternately hard and soft characters, has resulted 

 in producing a peculiar, abruptly roughened surface — narrow valleys 

 with abrupt, bluffy sides, and irregular table-like outliers. 



These peculiarities of topography, resulting from the different ef- 

 fects of the eroding agencies on the different kinds of rocks, have, 

 however, in a large portion of the region, been much modified and 

 obscured by other causes. I refer to those causes which gave rise 

 to theglacial drift. The southwestern quarter of the state of "Wis- 

 consin is without trace of the glacial depositions, whilst all of the 

 rest of the state is characterized by the presence of unusually largo 

 quantities of these materials. The line of demarkation, moreover, 

 between the drift-bearing and driftless areas, is a sharji and easily 

 traced one. Entering the state from the south on the southern line 

 of Green county, tlie drift limit traverses this county centrally from 

 south to north, and continues northward through western Dane and 

 central Sauk; then, curving eastward across the southern end of Adams, 



