TREATMENT OF THE COMSTOCK OEES. 209 



cast iron, correspoudiiig in form to the shoe of the stamp that falls upon it. It 

 is from 4 to 6 inches high. In the bottom of some mortars there are circular 

 recesses made for the reception of the dies which are caused to fit into them. 

 In others, to prevent the rock from working in under the die and displacing it, 

 the circular recess in the bed-plate is cast with a flange, and the die with a 

 small projection or lug. A groove is also made in the bottom of the mortar, 

 so that the die may be introduced with its lugs dropping into the groove. 

 The die being then turned about 90 degrees, the lugs come under the flanges 

 of the recess and the die consequently held in place. A simpler and the most 

 common form is to cast the cylindrical part of the die on a flat, square base, as 

 shown in Fig. 1, Plate XX. The bottom of the mortar is also made flat and the 

 dies dropped in, resting on their bases, which just fill up the space in the bot- 

 tom of the mortar. The corners of the bases of the dies are beveled off" so 

 as to allow the insertion of the point of a pick, by which means they can be 

 taken out when necessary. 



In addition to the dies, plates of iron, a half-inch thick, are sometimes 

 applied to the sides and ends of the mortar, exposed to constant wear, which, 

 like the dies, can be taken out and renewed when necessary. The top of the 

 mortar is covered by two pieces of plank, cut so as to fit closely, and resting 

 on flanges cast on each end. Semi-circular recesses, cut opposite each other 

 on the adjacent edges of the two pieces of plank, afford a passage for the 

 movement of the stamp-stems. 



The stamp consists of a stem or lifter ; a head or socket, attached to the 



lower end of the stem, and furnished with the shoe, a movable part which 



sustains the force of the blows and the wear of the operation ; and the coUar, 



or tappet, by means of which the revolving cam lifts the stamp for its faU. 



The stem is a round bar of wrought iron, about three inches in diameter, 



usually turned in a lathe. Its length is 10 or 12 feet. Its lower end is 



slightly tapered and corresponds in form to a socket or conical hole in the 



upper part of the stamp-head. The rest of the stem is usually made round 



throughout its entire length, the method, now in general use, of attaching 



the tappets to the stems not requiring any modification in the form of the 



latter, as was formerly the case. 



The stamp-head, illustrated by Fig. 2, Plate XX, is a cylindrical piece 

 27 



