IFTRODUCTORY AND HISTORICAL. 649 



in the general direction of the dip, in order to show the structure, ar- 

 rangement and irregularities of the strata. 



Barometrical Observations. Considerable time has been devoted 

 to ascertaini 11% the relative heights of different parts of the country, 



o O *- * ' 



for the purpose of ascertaining the amount of denudation, and pre- 

 paratory to making geological sections. These observations con- 

 sumed a great deal of time, and were made with much care; the plan 

 followed being as follows: A series of repeated observations were 

 made at points about half a mile apart, from the Illinois Central rail- 

 road at Scales Mound, to Calamine, on the Mineral Point railroad, and 

 from there to Mineral Point. A similar series was then carried from 

 there to Lone Rock, on the Chicago, Milwaukee and St. Paul railroad, 

 and from there to Lavalle, on the Chicago and Northwestern railway. 

 In the same manner a series was carried from Mineral Point through 

 Lancaster, to Glenhaven, on the Mississippi river, and from Mineral 

 Point east, to Moscow, and thence north, over the Blue Mounds, to 

 Mazomanie; and also from Moscow to Oregon, on the Chicago and 

 Northwestern railway. Starting from these known elevations, re- 

 peated observations were taken; and the mean of several series of 

 elevations, differing but little from each other, was assumed -to be 

 approximately correct. Then, from the various points on those lines, 

 series of lines were traveled over, embracing observations in all the 

 accessible points in the Lead region. 



The instrument used was a three-inch aneroid barometer, imported 

 by Wm. J. Young & Sons, of Philadelphia; a very delicate instru- 

 ment, and one which indicated differences of elevation with great 

 readiness and accuracy. The accuracy and value of barometrical ob- 

 servations depend greatly on the state of the weather at the time they* 

 are made. The summer seasons have been, as a general thing, quite 

 favorable for such observations, and in cases where it was not, allow- 

 ance has been made in computing the elevations. They may be con- 

 sidered as correct within a few feet. For the extent of this part of 

 the work the reader is referred to the topographical maps, which 

 show what has been done much better than it can be described. 



The value of a work of this kind is incalculable. By means of the 

 elevations obtained, we are readily enabled to make sections of any 

 part of the country, in any direction; showing the geological position 

 of the strata, with their dip, thickness, and the amount of denudation 

 at any particular locality. In mining localities this is especially val- 

 uable. It shows the position of the strata and openings, with their 

 dip; the elevation of the neighboring streams, and the practicability 

 of drainage, by means of levels; the depth to which shafts must be 



