16 GEOLOGICAL SURVEY OF WISCONSIN. 



TopoGEAPHicAL Sdeyev. - Much of the success of any geological 

 survey depends upon the accurate measurement of the elevation of 

 the country above some well known datum plane. For all such deter- 

 minations in Wisconsin the level of Lake Michigan forms a conven- 

 ient base from which to measure. The elevation of this lake above 

 the sea was long ago determined, by the topographer of the early 

 Michigan geological survey, under the direction of the lamented Dr. 

 Douglas Houghton, to be 578 feet. Probably the first spirit level 

 brought to Wisconsin was that of the late Hon. Byron Kilbourn, who 

 in March, 1836, established the zero, at the lake level, to which all 

 subsequent work of street grading, sewers, and the water-works of 

 Milwaukee has been referred. It was also from this now well es- 

 tablished datum plane that the surveys of the Milwaukee & Missis- 

 sippi, the La Crosse & Milwaukee, and the Milwaukee & Horicon 

 railroads were commenced; and it is to this zero therefore, that all 

 work of the present geological survey will be referred. 



Since 1836, the level of Lake Michigan has varied, from 4 feet above 

 this datum, in July, 1838, to 1.65 below in March, 1848, showing a 

 total change of 5.65 feet; the mean being about one foot above the 

 established zero. 



These fluctuations have long been known to result from various 

 causes, among them the effect of winds and storms; the annual change 

 of the seasons producing high and low water, as in ordinary rivers ; 

 the variations, during different years, or series of years, as to wetness 

 or dryness; and finally a regular (though very small) lunar tide, cor- 

 responding with that of the ocean. 



Eaileoad Elevations. — ■ Li the prosecution of any topographical 

 as well as of any other survey of the state, it is apparent that advant- 

 age should be taken of any work already done by others; and with 

 this view, efforts have been made to secure, as far as can now be done 

 the results of the several railroad surveys within the state, and the 

 following list of stations, with their elevation above Lake Michigan 

 and the sea will show what progress has been made in this direction. 



Every railroad station, the elevation of which is here given, becomes 

 a bench mark from which our barometrical measurements of the rela- 

 tive elevation and depression of various parts of the state, as well as 

 all future surveys can be made, with definite knowledge of the abso- 

 lute height above Lake Michigan and above the sea level. 



The level of Lake Michigan, thus definitely made known, will be 

 found a convenient datum plane to which all future railroad surveys 

 may be referred ; this can be done without the use of negative quan- 



