No. 2.] SPENCER — COPPER-MIMING. 73 



having a pressure of 60 pounds per square inch. Recently 

 diamond drills have been introduced into the copper mines on 

 Lake Superior for exploration purposes ; one being used in the 

 Quincy Mine at a depth of 1600 feet for sectional examinations 

 in place of the expensive cross-cutting. When we consider that 

 none of the native copper mines (except the Calumet and Hecla) 

 yield an average of two per cent of metal for all the rock broken, 

 and also the great expense entailed in sinking and drifting 

 through much poor rock, the only way to keep the mines success- 

 fully iu operation is to open up two or three year's galleries ahead 

 of the work, in order that when one remunerative area is 

 exhausted, another may at once be ready for stoping, and thus a 

 very considerable item in the annual expense of mining is expend- 

 ed without any immediate returns. 



Sometimes the hanging walls are so strong that only pillars, 

 at considerable distances apart, are required to support the roofs. 

 But in 1872, a sad accident occurred on the Ash-Bed at Copper 

 Falls. The pillars which had been left in part of the mine were 

 insufficient, and a portion of the roof having an area of 200 by 

 300 feet fell, killing six or seven men, and entrapping others for 

 several hours till released. However, this catastrophy could 

 scarcely have been unexpected, for the pillars had been slowly 

 but surely crushing and scaling off for more than a year, besides 

 giving more recent indications. Seldom do large masses fall 

 without giving warning, but the lives so exposed to danger come 

 to be held cheaply, and work is often pursued in spite of every- 

 thing, till either an accident happens, or at last prudence com- 

 pels the dangerous parts to be abandoned, or the miners to be 

 protected as much as possible. For this purpose I have known 

 the openings to be completely filled with crib-work of timbers, at 

 a very considerable expense. 



The workings of the veins and beds seldom exceed a width 

 or height of from six to ten feet, although the seams sometimes 

 widen to twenty feet or more. The wider veins are often 

 poor in copper, while in the beds, the metal is usually confined 

 to the upper portion of the amygdaloids or conglomerates. The 

 veins have generally well marked walls, sometimes slickensided, 

 while the metallic portions of the beds do not usually have their 

 limits well defined, or at least their foot-walls, and the metallic 

 zones sometimes leave their primary direction and wander off 

 into the lower parts of the beds. 



