112 MINING INDUSTET. 



and 20 or 30 feet in diameter, the roof, during excavation, being sustained by 

 a few posts and plank. When the chamber has attained the desired dimen- 

 sions these sHght supports are removed. The roof and sides soon begin to 

 swell and fall in, supplying the material which is wheeled out and dumped 

 into the stopes. The loose material being removed from the chamber, the 

 swelling and falling continues for an indefinite period, affording a supply for a 

 long time. 



Stope Timbering. — The difficulty of sustaining ground of this nature, by 

 any method of timbering, is not only in itself great, but is much increased by the 

 large size of the chambers rendered vacant by the extraction of the bodies of 

 ore. In the upper portions of the lode, worked several years ago, these cham- 

 bers were much greater in extent than now. One in the Gould and Curry, 

 400 or 500 feet in depth and length, is said to have been over 80 feet wide in 

 places, while several others had a width of 45 or 50 feet. At that time, more- 

 over, nearly all of the quartz was removed in working and no provision made 

 for filling the stopes with waste material to replace the ore extracted, leaving 

 these immense spaces to be kept open by timbering, which, to be efficient, even 

 for a time, needed to be of the most substantial sort. Methods ordinarily in 

 use in veins of moderate width and in firm rock were found to be insufficient. 

 To meet the necessities of the case a method of timbering was introduced, which 

 is said to have been devised by Mr. Didesheimer, then of the Ophir mine 

 which, though meeting with some opposition on account of its great cost, has 

 since been generally adopted and is now used by all the mines on the lode. 

 This consists in framing timbers together in rectangular sets, each set being 

 composed of a square base, placed horizontally, formed of four timbers, sills, 

 and crosspieces, 4 to 6 feet long, framed together, surmounted by four posts, 6 

 to 7 feet high, at each corner, and capped by a frame-work similar to that of 

 the base. These cap-pieces, forming the top of any set, are at the same time 

 the sills or base of the next set above, the posts, as the sets rise one above the 

 other in the stope, being generally placed in position directly over those below. 



This somewhat complicated system of timbering may also be described, 

 in other terms, as a succession of horizontal floors, composed of timbers that 

 are framed together in rectangular sets, 4 to 5 feet square, the floors being 

 supported one above the other by posts 7 to 8 feet high. Fig. 2, on Plate IV, 



