78 BULLETIN 42, UNITED STATES NATIONAL MUSEUM. 



them into suitable pieces so that tliey can be removed. Many arrange- 

 ments have been tried for this purpose, but the only one that has been 

 at all successful is to cut the masses up by means of chisels and ham- 

 mers. This is a long, laborious, and expensive operation. 



At first these masses absorbed entire attention, and no attempts what- 

 ever were made to utilize the copper that was found disseminated in 

 small amounts in so much of the rock of the country; but after the dis- 

 appointments of mining the masses more and more attention was 

 given these disseminated ores, and methods for the extraction of copper 

 were developed until they were made very profitable. These ores con- 

 tained copper in nodules, bunches, strings, and sheets, from micro- 

 scopic size up, and the fact that the percentage in the average rock is 

 small is compensated for by treating very large amounts of the mate- 

 rial. When the average percentage is about 1 in the soft amygdaloid 

 ores 175,000 to 200,000 tons of rock are treated annually from single 

 mines. The harder and conglomerate ores, like the Calumet and llecla, 

 yield a higher per centage of copper, but the cost of treatment is greater. 



The general plan of treatment is the same at all of the mines. After 

 the ore is taken from the mine a small amount of barren maierial is gen- 

 erally selected by hand and rejected. The ore is then crushed to a 

 suitable degree of fineness and the copper separated from the rock by 

 jigging, which is simply keeping the material agitated in a stream of 

 water so that the heavy copper will settle out, while the lighter rock 

 material will float away. 



For convenience the mines are divided into three classes, according 

 to the three characteristic occurrences of the metal. 



First. The so-called mass mines, which are characterized by the occur- 

 rence of large masses of free copper, amounting in some cases to many 

 tons of metal in a single mass. Besides these large masses these mines 

 also carry considerable disseminated free copper, generally in a true 

 vein. 



Second. The amygdaloid mines, which are characterized by the occur- 

 rence of the free copper in amygdules, bunches, strings, and sheets 

 from the size of a pin point up to a few hundred pounds in weight (with 

 rarely a hirge mass) disseminated in a soft amygdaloid trap rock. The 

 average percentage of copper in the ores from these mines varies from 

 three-quarters of 1 to 2 per cent. 



Third. The conglomerate mines, which are characterized by the occur- 

 rence of free copper, mostly in strings, in a hard conglomerate of ferru- 

 ginous quartz pebbles. The average percentage of copper in the ores 

 from these mines varies from 4 to 6 per cent. 



The occurrence and mining of free copper in the Lake Superior region 

 is very fully illustrated. A representative mine of each of the three 

 varieties was selected and collections taken from each. The collections 

 are designed to show fully the occurrence of the copper together with 



