ANNUAL REPORT FOR 1874. 57 



particular quarter section of land to these formations can be seen, and 

 its value with, respect to mining prospects at once ascertained. The 

 information from which these maps were constructed has been col- 

 lected with great care, involving an immense number of barometrical 

 observations, and a great many miles travel. 



From information contained in these two series of maps, it is easy to 

 construct geological sections on any desired Une. Those prepared by 

 Mr. Strong, and accompanying his report, have been selected with a view 

 of showing the " dip, number, magnitude, order and relative position 

 of the various strata," as well as the amount of denudation to which 

 the country has been subjected. The amount of denudation thus 

 shown, seems wonderful, but is accounted for when we consider the 

 immense time during which the rocks have been exposed to the abrad- 

 ing influences; being no less than the time occupied by the formation 

 of the Devonian, Carboniferous, and all subsequently deposited rocks. 



The regularity and simplicity of the geological features of the lead 

 region are quite remarkable; there being no sudden breaks or " faults " 

 in the strata, no upheavals to any considerable extent; no intrusive 

 rocks, breaking through the different formations; and the veins from 

 which ores are extracted do not penetrate the strata below the horizon 

 of the St. Peters sandstone. 



The origin and nature of springs are not well understood, many 

 crude notions being commonly entertained in regard to them; hence 

 Mr. Strong has, very properly, adduced the facts to show that the 

 springs near the top of the Blue Mounds have an abundant source of 

 supply, even in the small area of the mound above. 



Though the lead region is supposed to have been exempt from the 

 influence of the glaciers which have distributed so much drift mate- 



O 



rial over adjacent districts, there are some facts still requiring explan- 

 ation, particularly the one first noticed by Prof. Whitney, of the occur- 

 rence of blocks of St. Peters sandstone resting upon formations of 

 later age. The boundary of the glacial drift through Green county 

 has now been accurately traced. The occurrence of drift material in 

 the valleys of the Mississippi and lower Wisconsin is rightly attrib- 

 uted to river transportation from above. 



With respect to the several geological formations, much informa- 

 tion is given in detail, systematically arranged, and not before pub- 

 lished. 



Mr. Strong's survey, and the experience of the past twelve years 

 have given additional proof of the correctness of the views of Prof. 

 J. D. Whitney, as set forth in his report, published in 1861. Among 

 the most important of these views are the following: 



