MUSEUM OF COMrARATIVE ZOOLOOY. 107 



ores, especially those of copper, silver, and antimonial, arsenical, and 

 sulphurous ores of lead, or of lead with silver or copper. The fusing 

 points of pure copper and silver are below 2,500° ¥., that of copper 

 being, according to lliemsdyk's experiments, at the Utrecht mint, in 

 1869, 2426° F. and that of silver, 1904° F.; and hence these might have 

 passed into the melted rock in the liquid state; but whether this was 

 the fact, or whether they were in vapor, or in some vaporizable or solu- 

 ble compound, is not definitely known. 



" The above is a general explanation of the initial movement in the 

 making of copper mines like those of Lake Superior, in which the 

 metal is in the native state, and the silver mines of Nevada, Mexico, 

 Bolivia, Chili, Transylvania, and of many other regions, which aftord 

 various ores of silver with often some native silver. The igneous rock 

 of the Lake Superior region is largely doloryte, and copper is in fissures 

 and cavities in the igneous rock, and in the sandstone of the walls." 



In the third volume of the Geology of Wisconsin the copper-bearing 

 rocks are regarded as older than the sandstone, but younger than the 

 Hurouian. The traps, with their associated sandstones and conglom- 

 erates, are called the Keweenawan series, while the supposed unconform- 

 able sandstone is regarded as Potsdam. The evidence advanced is good, 

 so far as it goes, but it proves nothing iintil it shall be shown that the 

 old basalts studied in each particular case are not dikes, but overflows 

 identical in age with those on Keweenaw Point, and that they are not 

 earlier or later eruptions. In this respect the strongest evidence ad- 

 vanced by the Wisconsin geologists is fatally defective. Their methods 

 of observation fail in giving the proof necessary to establish their con- 

 clusions, which may or may not be correct, so far as their work goes. 



Historical Summary. 



The theories advanced concerning the Copper district are so various 

 and conflicting, in many cases even in the writings of the same author, 

 that we cannot hope to do justice to them in a brief summary. 



The principal points to which we have directed attention thus far are : 

 1st, the origin of the traps and their interbedded sandstones and con- 

 glomerates ; 2d, the relation that the traps bear to the eastern and 

 western sandstones ; 3d, the age of tlie eastern sandstone ; 4th, the 

 origin of the veins and the copper deposits. 



Concerning the origin of the traps, it has been seen that they were 

 said to be in dikes, generally intruded through the series of sandstones 



