KAOLIISr IX WISCONSIN. 15 



highest levels, whilst the irregular upper sui-face of the gueissic 

 rocks is apt to bring them up through the sandstone at any place. 

 A geological map, including Portage, Wood, Clark, and Jackson 

 counties, would show on the south the sandstone as the surface 

 formation, on the north the crj^stalline rocks, whilst where the 

 two meet the}' would be shown dovetailing into each other, the 

 Archaean extending many miles south in the stream beds, the sand- 

 stone penetrating as far north on the divides. As we trace the 

 rivers southward towards where the last crystalline rocks are seen, 

 these are found confining themselves more and more closely to the 

 vicinity of the streams until they are finally restricted to their 

 beds, the sandstone forming the banks. Thus the Wisconsin 

 River, for ten miles above Point Bass, and the Black for a greater 

 distance above the falls, present strips of cr^-stalline rocks only 

 as wide as their own currents. 



Another feature in the geology of the kaolin district seems 

 worthy of notice in the present connection. I refer to the fact that 

 the boundary line between the "driftless " area of the south west- 

 ern quarter of the State, and the " drift-bearing " area to the north 

 and east, crosses the district in a nearly east and west line from 

 Grand Rapids to Black River Station, on Black river. 



Nature cnid mode of occurrence of the Wisconsin kaolin. — The 

 Wisconsin kaolin occurs entirely as '" kaolinized " rock. As al- 

 ready stated it has been noticed only in the vicinity of the large 

 streams. This is so because elsewhere the crystalline rocks are for 

 the most part covered by the sandstone. Nearly always it occupies 

 exactly the original position, retaining sometimes even the minute 

 structure, of the unaltered rock. A fev/ cases were noticed im- 

 mediately on the river banks, where the structure of the clay 

 seemed to have been modified slightly by water action. The rocks 

 from which the kaolin has been formed, and into which it can fre- 

 quently be traced throush every degree of alteration, are beds inter- 

 stratified with the series of Archaean strata which have over wide 

 areas a common strike. Onlj^ the out-cropping edges of these beds 

 are decomposed, and as a consequence it follows that the resulting 

 kaolin forms narrow bands crossing the country in straight lines 

 parallel to the general strike. It is exceedingly common to find 

 overlying the kaolin a few layers of sandstone, sometimes a few 

 inches only, at others, a score or so of feet. In such cases the 



