270 REPORT OF NATIONAL MUSEUM, 1884. 



Ijossible to show every occurrence of each variety of au ore, neither lias 

 it been possible to represent every mining region. 



The Lake Superior copper region is very thoroughly represented, 

 both on account of the value of the mines of this region and as repre- 

 senting the kind of collections it is desirable for the Museum to possess 

 to illustrate a region or a mine. 



Taking, first, the region, it is represented by three prominent mines 

 showing three different and characteristic occurrences of the ore. 



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, are represented by the Central mine. 

 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 trajj-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 by the Conglomerate Mine. 

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

 from 4 per cent, to G per cent. 



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

 eral character of the ore and the inclosing wall rocks; secondly, 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 exhibit those that 

 are especially handsome or rich, but rather 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 plan of representation of this kind should be 

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

 Museum i^urposes than the haphazard collections of showy specimens 

 usually found in such establishments. 



COLLECTIONS IN ECONOMIC GEOLOOY. 



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. 



