342 DYNAMICAL GEOLOGY. 



ered into fissures, and amygdaloidal and other cavities disseminated through 

 the amygdaloidal rock ; and under such conditions they have been mined in 

 the Keweenaw copper region to the great depth mentioned. 



At Leadville, and other like regions, the liquid lavas were in part the 

 carriers of the ores and vapors to the surface ; but the chief part of the 

 concentration of the ores and the corrosion of the limestone may have taken 

 place during the cooling of the lavas. The solid rocks of the globe take in 

 their small percentage of moisture from the waters that become subterranean, 

 and then hold it ; a flow of such waters downward through such rocks, and a 

 draining out of their ores, cannot take place, except as complete decomposi- 

 tion is produced ; and the small depth to which decomposition extends in 

 most igneous rocks shows that the process is extremely slow. The processes 

 of decomposition and concentration were long kept in progress by the vapors 

 that continued to rise from below after the eruption had ceased. Finally, 

 the infiltration into the vein, or vein-masses, of cold waters from above has 

 carried on further the work of alteration and corrosion, and this work is still 

 in progress. 



3. Ore deposits of doubtful origin occurring in limestone. — Great lead 

 deposits occur in Paleozoic limestones of the Mississippi Valley in Wis- 

 consin, northern Illinois, and Iowa, and in Missouri and bordering parts 

 of Kansas and Arkansas. They occupy cavities or caverns in various lime- 

 stones from the Cambrian to the Subcarboniferous. The mines of Wisconsin 

 and Illinois are in the Galena limestone (or the upper part of the Trenton 

 limestone) of the Lower Silurian ; those of southeastern Missouri, in the 

 Third Magnesian limestone, of Cambrian age ; those of southwestern Mis- 

 souri, in the Keokuk limestone of the Subcarboniferous period, and to a 

 small extent in the Cambrian ; those of central Missouri, chiefly in the 

 Cambrian limestone, but partly in the Subcarboniferous limestone. 



The lead ore, galena, is associated with pyrite, marcasite ; the zinc ores, 

 calamine (zinc silicate) and smithsonite (zinc carbonate); lead carbonate, 

 malachite, barite, and in some places with black cobalt and an ore of nickel. 



The ore, in each of the regions mentioned, occurs in cavities or caverns 

 in the different limestones. From the resemblance between the various 

 deposits, it is concluded that the time of origin was the same for all, and 

 not earlier than the Subcarboniferous period, the age of the latest of the 

 limestones. 



As first made known in the geological report of Wisconsin by J. G. 

 Percival (1858), the ore-bearing cavities follow the courses of the joints (or 

 system of fractures) in the limestone, and are most extensive along the 

 larger joints, which are sometimes the lines also of faults. This fact has 

 been confirmed by later observations. 



In the Transactions of the St. Louis Academy of Science for 1875, A. 

 Schmidt announced the conclusion that the ore-containing cavities in the 

 Missouri limestones were made when the alterations of the galena took 

 place, producing the associated minerals, and principally in the more porous 



