684 GEOLOGY AND TOPOGRAPHY OF TEE LEAD REGION. 



The Galena limestone is the prevailing surface rock in the Lead 

 region. Its northern outcrop conforms closely to the main water- 

 shed, being parallel to, and always within a few miles north of it. 

 Its surface area is given as follows, for each range in the Lead region : 



190 sq. miles. 

 179 

 - 189 

 164 

 103 

 144 



969 



For a more particular exposition of the surface covered by this 

 formation, reference is made to the maps accompanying this report. 



Lithological Characteristics. The Galena limestone is almost in- 

 variably a very compact, hard, crystalline rock, of a yellowish gray 

 color, with numerous small cavities, sometimes filled with a softer 

 material, and sometimes lined with small crystals of calcite. The 

 upper portion is usually thick-bedded and free from flints, the layers 

 being from one to four feet thick, while the lower portion almost in- 

 variably consists of several feet of layers from one to two inches thick. 

 Good exposures of parts of this formation are frequently to be met 

 with; it may be seen in cliffs and ledges on nearly all the streams in 

 the Lead region. It always weathers irregularly in these natural ex- 

 posures, leaving the surface full of small cavities due to the removal 

 of the softer parts. 



The formation is characterized by layers of flints which however 

 are not constant in their occurrence in the same beds at difl'erent local- 

 ities. In some places there are several beds of flints,, which seem to 

 be connected with the openings, and serve as a guide to them, while 

 in others they are either entirely absent or occupying very different 

 geological positions. The flints are sometimes found in separate 

 layers, deposited conformably in the rock, and often in irregular 

 pieces distributed through the strata. .They seem to be confined 

 principally to the middle and lower parts of the formation, although 

 not entirely absent from any part. 



The general features of the formation will be more readily under- 

 stood by reference to the following descriptive section taken from a 

 bluff on the Mississippi river, situated on the S. E. qr. of Sec. 28. T. 

 3, E. 5 W., where the Potosi road leaves the valley: 



1. Heavy-bedded, Galena limestone; hard and compact, showing a crystalline struc- 



ture; stratification very regular; good buildmg stone, contains Heceptaculites . 2C 



2. Thin bedded, Hght yellow limestone, containing flints intercalated, and in layera 



between the beds g 



