€18 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [2] 



Besides these large masses these mines also carry considerable dissem- 

 inated free copper. 



Second, the Amygdaloid mines, which are characterized by the oc- 

 currence 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 large mass) disseminated in a soft amygdaloid trap-rock, are 

 represented by the Osceola mine. 



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

 from three-quarters of 1 per cent, to 2 per cent. 



Third, the Conglomerate mines, which are characterized by the oc- 

 currence of the free copper mostly in strings in a hard conglomerate of 

 ferruginous quartz pebbles, are represented b^^ the Conglomerate mine. 

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

 from 4 per cent, to 6 per cent. 



Taking the Conglomerate mine, the collection shows, first, the gen- 

 eral character of the ore and the inclosing wall rocks ; secoudly, it shows 

 the occurrence of the ore at various prominent points in the mine which 

 are accurately located ; and, thirdly, it shows a section of the rocks over 

 a distance of 631 feet, by specimens taken at suitable distances to show 

 the different characters and changes of the material. 



In selecting specimens it has not been designed to take those that are 

 especially handsome or rich, but ratber to take such as represent the 

 actual character, occurrence, and value of the ores. In making col- 

 lections of ores for the National Museum, it is very desirable that some 

 definite and systematic i)lan of representation of this kind should be 

 adopted, as collections made in this way have far more value for 

 Museum purposes than the haphazard collections of showy specimens 

 .usually found in such establishments. 



COLLECTIONS IN ECONOMIC GEOLOGY. 



Gold. 



Placer gold, from Virginia, North Carolina, California, Idaho, Mon- 

 tana, Utah, and Oregon. 



Gold quartz from Virginia, North Carolina, South Carolina, Georgia, 

 California, and Montana. 



Auriferous gravel, from California and South Carolina. 



Auriferous pyrite from Virginia and Colorado. 



Telluride ores — compounds of gold with tellurium from Colorado. 



Iridium. 



Iridosmine, from California. 



Silver. 



Native silver with native copper, from the Lake Superior region. 

 Native silver on sulphide of copper, from Montana. 



