502 GEOLOGY OF CENTRAL WISCONSIN. 



All the areas, except the one, or rather the group, including the 

 Baraboo ranges in Sank county, are of small size, generally occupying 

 much less than a square mile of area. With the same exception, they 

 are all mound-like in form, rising, usually, somewhat abruptly from 

 the surrounding country, which is frequently level, and showing al- 

 ways considerable rock exposures on the flanks and summits, being 

 often almost all of bare rock. They reach heights of from 50 to 250 

 feet, but are usually lower than surrounding outlying bluffs of the 

 horizontal strata. The Baraboo group, unlike the others, constitutes 

 a series of bold ridges, one of which reaches elevations of 800 and 900 

 feet above Lake Michigan, and a length of over 20 miles. These 

 ranges are so important an element in the topographical features of 

 Central Wisconsin, that they have already received attention in the 

 chapter on general topography. They are still more fully described 

 in subsequent pages. 



The nature of the rocks composing the several areas is not always 

 the same. The large areas in Sauk county, and a few others, are 

 chiefly of quartzite; a number are of quartz-porphyry; still others 

 of granite, which is different in different cases; and yet others, occur- 

 ing in Jackson county, and close to the main Archaean area, are of 

 ferruginous quartz-schist. 



Except in the cases of the granitic areas, these rocks arc generally 

 quite distinctly bedded, and are usually tilted at high angles. 



In many of the areas, especially in those whose elevation is consid- 

 erable, horizontal sandstone is found lying immediately against the 

 tilted crystalline rocks, pebbles and boulders from which frequently 

 occur in the sandstone, giving it often a rough, conglomeratic charac- 

 ter, and proving at once the great antiquity and non-intrusive nature 

 of the rocks from which they are derived. Some of the areas have, 

 without doubt, been once entirely buried beneath the sandstone layers, 

 to whose subsequent denudation they owe their resurrection. 



The following tabulation gives, in a condensed manner, and in a 

 form convenient for comparison, the location, size, nature, etc., of each 

 of the known areas. The facts with regard to Nos. II, III, XI, XIII 

 and XVIII, are furnished by Prof. Chamberlin, in whose report de- 

 scriptions of them will be found. 



