COLLECTIONS IN ECONOMIC GEOLOGY AND METALLURGY. 79 



its associates, and includes several sections of the mines to represent 

 the clianges of the material in goin<? up and down and also across the 

 formation. To these mine collections are added full illustrations of 

 the dressing- operations and some illustrations of the smelting opera- 

 tions. 



THE CENTRAL MINE. 



The Central Mine was selected to illustrate the group of mass mines, 

 as it is one of the oldest as well as one of the most successful of these 

 mines. It Avas opened in 1854, and the first year of operation produced 

 over 67 tons of refined copper, and from that time on it has been regu- 

 larly i)roductive and profitable. It is located on the southern sloi)e of 

 the greenstone range, 5 miles from Eagle Harbor, on Lalie Superior, 

 from which the copper is shipped by boat. 



The vein, like many other veins of this region, varies greatly' in thick- 

 ness, being at times a to 6 feet wide, and in others dwindling down to 

 a few inches. Its general direction is north and south, and its general 

 dip towards the east. The dip, however, is small and varies, sometimes 

 being vertical, and sometimes even being towards the west. The filling 

 of the vein is quite complex, and varies greatly, the principal minerals 

 being calcite, quartz, epidote, and various zeolites. While the copper 

 .is principally found in masses, yet a considerable amount of dissemi- 

 nated copper is also extracted. The vein is most productive when dip- 

 ping towards the east, and is much poorer when dipping towards the 

 west or vertical. In some cases the copper is not confined to the vein 

 itself, but extends into the walls. The vein has cut through three 

 beds of conglomerate, and a large number of amygdaloid beds, most of 

 which contain some copper, but only one conglomerate has been worked 

 to any extent. The mine is opened by four vertical shafts, and a series 

 of cross-cuts to the vein. 



Whenever a mass of copper is discovered in the mine it is laid bare 

 over its whole extent on the under side, and it is then undermined suf- 

 ficiently to allovv^ it to be blown down by a blast behind it. After the 

 mass has been detached from the wall the rock material is removed by 

 sledges, and when necessary by blasting. When there is a considerable 

 amount of rock material scattered through the copper it is sometimes 

 possible to break up the mass by blasting with dynamite, but the solid 

 masses of copper must be cut up. The cutting tool consists of a bar of 

 sieel 2 inches wide and one-half inch thick, and of varying length, 

 according to the depth of channel to be cut. The point is drawn down 

 accross the width and fiared a trifle beyond the thickness in order to 

 give clearance. This tool is held by one workman while auotiier strikes 

 it with a six-pound sledge. In this way a thin chip of copper is cut 

 and the channel is deepened by successive cuts until a piece is sepa- 

 rated from the mass. Great skill is required to do this cutting properly, 

 and at the very best the progress is very slow. After a piece has been 



