INTRODUCTION. 



The following report covers a region having an area of about 10,000 

 square miles, and is the result of something more than nine months, 

 in all, of field work. The greater part of this was done in the seasons 

 of 1874 and 1875, my work during the other years of the existence of 

 the survey having been in the Lake Superior country, which will be 

 treated of in a subsequent volume. 



The district now reported on, including the central counties of 

 Wisconsin, is nearly one-fourth larger than the state of New Jersey, 

 whose third geological survey has been in progress from 1864 up to 

 the present time, and has issued, besides numerous annual reports, a 

 large volume of nine hundred pages, and an atlas of maps. Prof. 

 Geikie, director .of the geological survey of Scotland, has recently 

 made the statement, in a public lecture, that the average annual 

 amount of ground gone over by each geologist of that survey is about 

 one hundred square miles, this amount of labor being performed by 

 an average daily walk of ten to fifteen miles, in a year of two hundred 

 days in the field. At the same rate, an examination of the Central 

 "Wisconsin district would require over one hundred years of continu- 

 ous work. These statements are made in order that a fair judgment 

 may be passed upon the results accomplished, as compared with those 

 of the surveys of other states and countries, and in order that it may 

 be understood that no claim is made of having made an exhaustive 

 survey of the district reported upon. 



Geological mapping is accurate, ceteris paribus, exactly in pro- 

 portion as the geographical maps used as a basis are accurate. One 

 great advantage enjoyed by the surveys of Great Britain lies in the 

 almost absolute accuracy of the celebrated Ordnance Maps of that 

 country, which are drawn on a scale of six inches to the mile, and 

 show every topographical feature, road, and house, with such faithful- 

 ness that the geologist has little more to do than to mark upon them 

 outcrops as fast as found. In the United States, the only maps hav- 

 ing any genuine claim to accuracy are those of the coasts of the 

 continent, and of the shores of the great lakes, made by the gov- 

 ernment triangulation surveys, and even these are, for the most part, 

 on too small a scale to be of much use in geological mapping. Nev- 

 ertheless, in this regard, we have in Wisconsin a great advantage over 



