B, Silliman on the Grass Valley District. 241 
saving processes with a view especially to concentrating the 
sulphurets. These processes vary much in different mines. In 
some mills, especially the Ophir, much more elaborate mechani- 
eal apparatus has lately been introduced—with what results still 
remains to be seen. It is certain that if the method of treat- 
ment just sketched seems imperfect, (as it undoubtedly is,) it is 
the method which has hitherto yielded the large returns of gold 
for which Grass Valley has obtained its well-deserved renown. 
As the development of the: district goes forward, cases will oc- 
cur of veins containing gold in a state of very fine division, to 
which other methods of treatment must be applied. Suc 
é sulphurets occurring in the Grass Valley District are 
, Of the length of the productive portion of quartz veins and 
the depth at sees the eaukinioies to become productive, Grass 
alley offers some instructive examples. 
The North Star vein, on Weimer Hill, has been proved pro- 
ductive on a stretch of about one thousand feet, while the propor- 
. 
ton of gold has gradually increased with the depth, from an aver- 
* 
Yielding scarcely the cost of millin 
_ Smmon productiveness. An opinion : eee Set 
has been rhaps generally accepted by most writers on vag 
_ | ret goldcbearing quartz veins, that they were richest near the” 
Aw. Jour. Sc1.—Szconp Ssrtes, VoL. XLIV, No. 131.—Szpr., 1867. 
31 
