4,64 GEOLOGY OF CENTRAL WISCONSIN. 



felspathic rocks, and also those that are largely hornblendic. The alter- 

 ation usually extends but a small distance into the body of the rock, 

 which can generally be observed in its true unaltered character by re- 

 moving the surface crust. In some localities, however — and these are 

 altogether without the drift area, or at least where the drift materials 

 are scanty — the whole exjjosed portion of the rock is so disintegrated 

 as to crumble to dust under a blow of the hammer, or is completely al- 

 tered, by removal of alkaline ingredients and absorption of water, to a 

 clayey mass, which, when the original rock was non-ferruginons, or 

 when the iron oxides have been also completely removed, is a mixture 

 of pure kaolinite and quartz grains, and is of considerable commercial 

 value. The almost entire restriction, so far as known, of this kaolin- 

 ized rock to those districts where there is nothing to indicate the 

 former presence of the glacial agencies, is a fact of very considerable 

 interest. Its absence everywhere where the drift occurs may indicate 

 that it has been removed by the drift forces. It is true, however, 

 that the kaolin district coincides with one in which there is generally 

 more or less of a sandstone covering to the crystalline rocks, and that 

 many of the kaolin occurrences are beneath a few layers at least of 

 sandstone. It may then be that the surface waters, percolating 

 through the porous sandstone — in ancient times much thicker than 

 now — have formed natural watercourses along the junction between 

 it and the less easily penetrable crystalline rocks, and have thus ex- 

 erted an unusual disintegrating action; whilst the sandstone itself 

 has subsequently acted as a preserver of the kaolinized rock from the 

 ordinary eroding agencies. 



No one system of strikes prevails over the whole region, but yet 

 for long distances a marked parallelism can be observed in the courses 

 of the outcrops of the various layers. Thus along the Wisconsin 

 from its southernmost exposures of crystalline rocks, at Point Bass, 

 in Wood county, as far north as Wausau, in Marathon county, the 

 strikes are, for the most part, east of north, whilst the dips, though 

 of course far less constant in amount and direction, are more com- 

 monly north than south. On Black river, again, for the whole dis- 

 tance examined, the strikes are just as markedly northwest, and on 

 Yellow river more commonly west, than east of north. Whilst no 

 general system of strikes can be laid down for the whole region, and 

 no further generalizations can be drawn from the observations made 

 in the Central Wisconsin district, it is nevertheless very probable that 

 by comparing the results of diflferent observers on the strike direc- 

 tions for the whole Archaean region of the state, some quite import- 

 ant conclusions may yet be reached. At the time of writing investiga- 



