12 BULLETIN 42, UNITED STATES NATIONAL MUSEUM. 



Ill .some cases tbe gold is so imich conceutrateil into certain well-de- 

 fined strata that they can be removed without (lisiurbing the remainder 

 of the deposit, by a system of mining; known as drifting. 



NORTH BI.OOMFIELI) MINK. 



To illustrate the hydraulic process nine specimens are shown from the 

 North Bloomfield Gravel Mine, North Bloomtield, Nevada County, Cal- 

 ifornia, one of the largest hydraulic mines in the State. The collection 

 was made by Mr. W. H. Eadford, superintendent, October, 1884. 



(1) Auriferous gravel, average fair ([uality, from 1 foot above bed-rock. (55404.) 



(2) Nuggets, obtained in cleaning up. (55316. ) 



(3) Fine gold, from gravel lying directly on the bed-rock ; obtained by the drifting 

 process. (55315.) 



(4) Quicksilver or mercury, used to amalgamate and save tbe gold as it passes 

 through the sluices. Squeezed from amalgam. (55312.) 



(5) Quicksilver or mercury, used to amalgamate and save the gold as it passes 

 through the sluices. Distilled in retorting amalgam. (55313.) 



(6) Gold amalsam, taken from the -sluices in cleaning np. Puritied and squeezed 

 amalgam. (55314.) 



(7) Sponge gold, obtained by heating the gold amalgam to drive off the mercury. 

 (55317. ) 



(8) Black sand, consisting of various heavy minerals, especially magnetite, pyrite, 

 zircon, and cassiterite, obtained with the amalgam in the sluices when cleaning up. 

 (55318.) 



(9) Sulphurets and heavy material, obtained in the clean-up in working over and 

 purifying the amalgam. (55319. ) 



The operation of the process is further illustrated by the scheme as 

 shown in Fig. 1. 



THE EXTRACTION OF GOLD FROM SULPHURETED ORES BY STAMP- 

 ING AND AMALGAMATING. 



BOBTAII. MILL. 



This process is illustrated by a collection from the Bobtail mill. Black 

 Hawk, Gilpin County, Colorado. 



The ore as delivered to the mill is first passed over screens to sep- 

 arate the fine and coarse material ; the coarse material is crushed be- 

 fore being fed into the mortars, while the fine is fed direct. The mor- 

 tars consist of cast iron receptacles, provided with a suitable bottom 

 of cast-iron, that can be easily renewed. The sizes of the mortars vary 

 according to circumstances, especially as regards depth. The front and 

 back of the mortars are provided with amalgamated plates, to catch as 

 much as possible of the gold during the crushing. Above the plates 

 in the front of the mortar is an SOinesh screen. A stream of water is 

 kept tlowing through the mixture during the crushing. 



The ore being thrown into the mortar, the stamp is allowed to drop 

 upon it in order to reduce it to the proper size. The stamp consists of 

 a suitable piece of cast-iron on the end of a rod, that can be raised by 



