THE LOWER SILURIAN ROCKS. 563 



Lead Region and Eastern Wisconsin districts, in the reports on which 

 it will be found fully described, it is not thought necessary to give it 

 any attention here. 



II. Local Details. 1 

 POKTAGE, WOOD, CLARK, AND JACKSON COUNTIES. 



(ATLAS PLATE XV, AREA F.) 



The only one of the Lower Silurian formations occurring in these counties is the 

 Potsdam sandstone, which forms the basement rock of the southern portions of the 

 three first named, the Archaean rocks rising to the surface in their northern portions. 

 In Jackson county only the bed of Black river and a few scattering mounds show the 

 Archaean rocks. 



The peculiar irregularities of the line of junction between the two formations, the ex- 

 tension southward along the stream valleys of long strips of the crystalline rocks, the 

 corresponding northward extension, along the divides, of the sandstone, and the difficul- 

 ties met with in tracing the boundary, have been before alluded to. The facts upon 

 which the junction line for the region covered by the map of Area F. of the Atlas is 

 based, including the location of a number of outcrops, have also been given briefly, and 

 will not be repeated here. 



A very large proportion of the sandstone area in these counties is level, and is, to a 

 considerable extent, occupied by large marshes. Towns 21 and 22. ranges 7 and 8 east, 

 Portage county, are almost all included in one great marsh, as are also towns 21, ranges 

 2, 3 and 4 east, in Wood county, the latter marsh extending also over considerable por- 

 tions of the towns to the northward, and having a still greater extent into Juneau and 

 Jackson counties on the south. Underneath these marshes, which, to a large extent, 

 have peat bottoms, sandstone is commonly found at shallow depths. On some of the 

 dividing ridges again, the sandstone country becomes considerably elevated, and has 

 more or less a rolling character. Such is especially the case with the divide between 

 the Black and Trempealeau rivers in western Jackson county, which is without drift 

 covering, and is worn into the deeply ravined surface characteristic of driftless regions, 

 The divide between Black and Yellow rivers, in western Wood and eastern Clark coun- 

 ties, is considerably elevated above the surrounding country, but is very heavily coated 

 with glacial materials, and presents therefore a much more even surface. 



The larger part of the sandstone area of Portage, Wood, Clark, and eastern Jackson 

 counties, is within the region of heavy timber, chiefly pine. In the southern portions of 

 the three first named, and in a large part of western Jackson, small pines mingle with 

 the small oaks tha*~. are characteristic of nearly all of Central Wisconsin, the growth of 

 timber in all of these portions being scant and small, and associated with a loose, sandy 

 soil. On the northern part of the divide between Yellow and Black rivers, however, 

 the sandstone is deeply buried beneath clay drift, as a result of which we find excellent 

 clay soils, and a heavy growth of hard wood timber, for the most part maple. 



Usually the sandstone of these counties is but a thin covering upon the crystalline 

 rocks, which appear in all of the deeper stream- valleys. High bluffs of the sandstone, 



i In the manuscript, this division of the report includes a full description, by townships, of a 

 large part of the country occupied by the Lower Silurian formations, embracing topography, sur- 

 face features, rock outcrops, etc. It has been found necessary, in order not to erzcaed the limits 

 originally assigned to this report, to throw out most of this material, and a number of important 

 outcrops are therefore not alluded to. This omission can be, in part, made up for, by any one who 

 wishes further information than given, by a study of the Atlas maps in connection with, the lists of 

 altitudes of Chapter I. The whole amount of material thrown out would make ahont 45 pages of 

 the small typo. K. D. I. 



