B. Silliman on the Mining Districts of Arizona. 299 
in the fire and then quenched in water, leaving the surface 
deeply stained with red oxyd of iron; this stain penetrating 
phurets, This change is very conspicuous in the Techatticup 
lode in Eldorado caiion which has been open to a depth of 140 
feet, and shows, as I am informed by Henry Janin, a gradual 
the San Francisco District, next to the quartz and feldspar which 
form the great mass of the lodes, is fluor spar, a mineral fre+ 
quently seen elsewhere in the world as an associate in silver-bear- - 
ing lodes—as, for example, in Freiberg in Saxony—but which 
18 of rare occurrence in this country in a similar association. 
This mineral is found abundantly in the Skinner lode, the Day- 
ton, the Knickerbocker, and the Quackenbush; and has been 
Sserved also in the Moss and several others. It is associ 
them with free gold, horn silver sometimes in distinet dode- 
cahedral erystals, and iron gos é ue 
Description of some of the veins of the San Francisco District. 
In general the lodes in San Francisco District are remarkably 
_ Vertical, rarely deviating more than 80° from ndicu- 
