248 GEOLOGY OF EASTERN WISCONSIN. 



CHAPTEE YL 

 ARCHAEAN FORMATIONS. 



The district under consideration is occupied almost exclusively by 

 Paleozoic formations. The Archaean rocks form the great sloping 

 floor upon which these later deposits rest, and rise to the surface along 

 their northern border. But the Archaean surface is very irregular, 

 and here and there knobs rise through the superincumbent formations, 

 giving rise to isolated hills of quartzite, porphyry and granite in the 

 midst of the areas of later rocks. It is to these protruding, but not 

 intrusive, masses that our attention will be chiefly confined. 



Along their northern border, the Paleozoic formations lap upon an 

 immense series of granitic and allied rocks, that will receive due con- 

 sideration in the several reports upon the Archaean regions. They are 

 here referred to as a convenient point of departure in describing the 

 isolated areas above referred to, whose main importance depends upon 

 their position with reference to this Archaean region, and their rela- 

 tions to subsequent formations. 



THE MUKWA GRANITE. 



The isolated outlier found in the S. E. qr. of the K. E. qr. of Sec. 

 26, and the N". W qr. of the S. W. qr. of Sec. 25, town of Mukwa, 

 Waupaca county, lies nearest the main Archaean area. This outcrop 

 seems to have been unknown to the geologists heretofore, and came 

 to my attention through information derived from Mr. Carr, of New 

 London. 



It consists of three large, and as many small, rounded, elongated, 

 dome-like outliers, arranged nearly in a line trending "W. 35 to 40 

 ]ST., and rising near the center to a height of nearly 70 feet. 



The rock consists chiefly of red feldspar, with which is associated a 

 less quantity of quartz and a small and varying amount of a dark 

 mineral, which was not seen in the distinct crystalline form, but 

 which seemed to be an aggregation of minute blended blades of bio- 

 tite. The crystals of feldspar are never large, seldom exceeding a 

 quarter of an inch in length, and are usually quite minute, so that 

 some portions, from which the dark mineral is absent, closely resemble 



