THE ARCELEAN ROCKS. 461 



CHAPTER III. 

 THE ARCH^AN ROCKS. 



THE MAIN ARCH^AN AREA. 



I. In General. 



The crystalline rocks of the Central Wisconsin district may be con- 

 veniently separated into two groups, for the purposes of description ; 

 those of the main Archaean body constituting one of these, and those 

 which occur in isolated protruding patches within the region of the 

 Silurian sediments, the other. The two are of course but parts of the 

 same grand mass, which everywhere underlies the undisturbed Silu- 

 rian beds, a fact proven, not merely by geognostical theory, nor the 

 evident passage of the crystalline rocks beneath the Silurian, but also 

 by the numerous Artesian borings which have penetrated through the 

 horizontal strata into the underlying Archaean, at points widely scat- 

 tered over the state. At present, however, we have to do only with 

 those of the crystalline rocks that appear at the surface. 

 The area occupied by the rocks of the main Archaean mass, so far as the 

 district at present under consideration is concerned, covers all of Mara- 

 thon, most of Wood, and much of Clark, Jackson and Portage counties. 

 The line of junction between the Archaean area and that of the 

 next formation to the south, the Potsdam, or Lower, sandstone, is 

 exceedingly irregular, and often quite difficult to trace. The sand- 

 stone is frequently found stretching far northward along the water- 

 sheds between the southward flowing streams, whilst the gneissic and 

 other crystalline rocks are to be observed stretching as far south along 

 the stream valleys, the areas of the two formations thus dovetailing 

 into one another. Thus the Wisconsin and the Black rivers make 

 rapids over tilted gneissic strata for many miles south of their first 

 entrance upon the sandstone region, and as their courses are followed 

 downwards the exposures of these rocks are to be found more and 

 more closely restricted to the stream limits, until they finally occur 

 in the river beds only, the sandstone overlying them in the banks. 

 A more definite idea of the position of the boundary line between the 

 sandstone and Archaean areas may be obtained from the general geo- 

 logical map of the state (Atlas Plate I), and also from Plate XY of 



