100 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
"brittle silver' 1 (?) was found in the shaft, but no pay ore was encoun- 
tered in a lower crosscut tunnel. 
From the foregoing it appears that in this region pay shoots con- 
taining chiefly ores of lead, silver, and copper may change in miner- 
alogical character and in value within very moderate depths. But 
while galena in one case has been succeeded by tetrahedrite and in 
another case partly by sphalerite, in other lodes containing silver- 
lead ore tetrahedrite (Royal Tiger and Buckeye mines) and sphal- 
erite (Iowa lode) have been more abundant in the upper than in the 
lower workings. Thus the facts, as at present known, do not justify 
the general statement that galena passes into gray copper with depth, 
or vice versa. The change from galena to sphalerite with increasing 
depth is a well-known phenomenon in the lead and zinc deposits of 
southwestern Wisconsin, and is explained by Van Hise * as a result 
(1) of a first concentration by ascending waters and (2) of a second 
concentration by descending waters. It is possible that the change 
recorded in the Belcher is of this character. But the occurrence of 
abundant argentiferous galena and tetrahedrite in the Virginius lode 
at the Revenue Tunnel level, over 2,000 feet below the croppings, and 
of galena in the deepest workings on the New York CHy lode, indicate 
that both tetrahedrite and galena may occur abundantly and retain 
their values at far greater depths than were reached in the Aspen, 
North Star, and other mines of the Silverton quadrangle, in which 
the pay shoots are reported to have diminished in value or to have 
disappeared entirely. As applied to the lodes, therefore, the often- 
heard arbitrary dictum that galena and tetrahedrite "do not go 
down," in so far as it relates to the depths ordinarily met with in 
mining, is a generalization based on insufficient premises, although, 
as will appear in the discussion of the origin of the ores, there are 
theoretical reasons for supposing these minerals less likely than cer- 
tain other sulphides to be found in abundance at great depths. With 
regard to the rich silver ores argentite and pioustite, the known facts, 
incomplete as they are, seem to warrant the conclusion that these 
minerals occur in the uppet' portions of the lodes only, and do not 
extend to great depths, probably rarely over 500 feet in this region. 
This is in general accord with observations in other districts where 
these and other rich silver minerals occur, and, as will be later shown, 
this fact has an important bearing on the question of the genesis of 
the ores. 
The vertical range of the argentiferous sulphobismuthites of lead 
occurring in the Silverton region is not known, but there are some 
indications that they do not extend to great depths. 
The free gold ores have not yet been worked to a sufficient extent 
in the Silverton quadrangle to furnish any very reliable data as to 
'Some principles controlling the deposition of ores: Trans. Am. Inst. Min. Eng., Vol. XXX, 
1900, pp. 104-109. 
