18 GEOLOGY OF ILLINOIS. 



this subject has already been discussed at some length. 

 The Wisconsin and Iowa State Geological Reports both de- 

 vote considerable space to the discussion of the causes of the 

 deposit of lead, its modes of occurrence, and various other 

 interesting thoughts and questions connected therewith. 



A brief reference to our present knowledge upon this 

 interesting subject of inquiry is all that will be attempted 

 at the present time. If deemed necessary it will be more 

 fully treated of in the report upon the geology of Jo Da- 

 viess county. 



The origin of the lead and other minerals of this district 

 is one of the vexed questions. At least four theories have 

 been advanced and argued. These are as follows: 



1. That the waters of the Silurian ocean held the min- 

 erals, or their salts, in solution. At the time of the depo- 

 sition of the lead-bearing rocks, or at least before the depo- 

 sition of the next overlying formation, the mineral matter 

 was precipitated, in the form of sulphurets, in the crevices 

 of the rocks. 



2. The injection from below of the mineral matter in a 

 melted state up through the crevices of the rocky mass. 

 Many dykes, lodes, and true mineral veins, thrown up by 

 volcanic or other igneous agency, doubtless owe their origin 

 to this source; but it does not satisfactorily explain the ori- 

 gin of the Galena lead. 



3. The theory of sublimation supposes the metal to 

 have existed in hot vapor or steam. As this cooled, crys- 

 talization took place, and the mineral matter adhered to the 

 sides of the fissures in the rocks, as frost crystalizes upon 

 the window-panes. Craters of volcanoes, flues of furnaces 

 and bloomaries also furnish familiar examples. 



4. Another theory supposes that electro-chemical action 

 caused a segregation of minerals into crystals in the soil, 

 as geodes are formed. 



The first of these theories is the most universally re- 

 ceived, so far as the origin of lead and its associated mine- 



