THE LOWER SILURIAN ROCKS. 529 



applying especially to the succession of strata exposed along the St. 

 Croix river. He notices also, distinctly, the bed I have called the Men- 

 dota, and also numbers of other minor points mentioned by no other 

 geologist. 



The surface distribution of the Lower Sandstone exceeds that of 

 any other of the formations of the Central Wisconsin district. The 

 region occupied by it embraces all of Juneau, Adams, "Waushara and 

 Marquette counties, as also more or less of Portage, Wood, Clark, 

 Jackson, Sauk, Green Lake, Columbia and Dane counties in all, an 

 area of over 6,000 square miles. The total area occupied by the for- 

 mation outside the Central Wisconsin district, and within the state, 

 is probably as large again, but nowhere else is there one continuous 

 area of so great diameter as in the central counties. Over a large 

 portion of this district, in Jackson, Wood, Clark, Portage, and por- 

 tions of Juneau and Adams counties, there is no distinct evidence 

 that the newer formations ever spread. Another large portion, in- 

 cluding Waushara, Marquette, northern Green Lake, western Colum- 

 bia, northern Sauk and southern Juneau, well away from the re- 

 gions of the newer rocks, was originally, beyond doubt, overlaid 

 by at least the Lower Magnesian, this formation occurring on two out- 

 lying bluffs in the northwest corner of Marquette county, 25 miles 

 distant from the main area of that formation. There are again small- 

 er areas, as the strip along the Wisconsin below the great bend, and 

 the district about the head waters of the Catfish, in Dane county, 

 which occur altogether within the country of the newer formations, 

 and have, beyond doubt, been reached by erosion along the existing 

 valleys. 



On the north, the main area occupied by the lower sandstone is 

 limited by the larger one in which the crystalline rocks are at the 

 surface. The boundary between the Potsdam and Archaean areas 

 is quite difficult to trace. As stated on a previous page, the streams 

 flowing southward from the Archaean area cut through the sandstone 

 beds down to the crystalline rocks, for many miles after entering the 

 sandstone district, whilst on the divides between the streams the sand- 

 stone stretches as far north into the Archaean regions. The difficul- 

 ties in tracing the boundary line in any greater detail than thus 

 stated lie in the peculiar irregularity of the upper face of the 

 older rocks, which may bring them to the surface at any point; in 

 the once greater spread northward of the sandstone, as a result of 

 which it is liable to be found in little patches, filling the depressions 

 of the older rocks; and in the heavy coating of drift that conceals the 

 rock beneath over considerable areas. To these difficulties may be 

 Wis. SUR. 34 



