RANSOM E,] 
STRUCTURE OF THE LODE ORES. 
89 
structure, not infrequently reveal a tendency toward a final crystal- 
lization of the galena in the middle portion of the vein, as may be seen 
in parts of the Iowa lode and in the Idaho claim, on the Titusville 
lode. This same tendency is also illustrated in fig. 8, which is a 
sketch of a small regularly banded vein prospected by the roadside 
half a mile north of Houghton Mountain and close to the London 
shaft. 
Remarkably fine and regular banding was observed in the ore 
thrown out on the dump of a small deserted tunnel on what is proba- 
bly the Osceola claim, in Cunningham Gulch, about half a mile above 
Stony Gulch. In its most perfect form this banding consists of dark 
sheets of finely crystalline sphalerite and galena about one-half milli- 
meter in thickness, separated by plates of vitreous quartz about 2 
millimeters in thickness. The result is a remarkably regular and 
striking fine banding (PI. XII, A). The little sheets of quartz fre- 
quently show comb structure and have apparently crystallized, in 
Fig. 8.— Cross section of banded vein near the London shaft, Mineral Point. «, country rock; 
6, quartz and chalcopyrite; c, tetrahedrite; <l, d', quartz; e, galena. 
open spaces. In some facies with rather wider banding chalcopyrite 
occurs, chiefly in the quartzose bands. The cause of such fine and 
regular banding is not known, but is probably connected with 
metasomatic replacement. The ore is apparently of too low grade 
for profitable working and was not seen in place. 
Another form of original banding is that exhibited by the rich gold 
Dre of the Camp Bird mine (PI. XII, B). As elsewhere described, the 
rich portion of this lode is a sheeted zone in San Juan anclesitic 
3reccia and lies next the main hanging wall of the lode. Within the 
ndividual fissures of this zone the quartz has crystallized freely in 
)pen spaces. Thus, in the specimen illustrated, which represents a 
'haracteristic cross section of one of the richest ore streaks, the quartz 
crystals are implanted at their bases on a thin seam of chalcedony and 
jarbonates carrying a little finely divided galena and sphalerite and 
associated with pulverulent white kaolin. The thickness of this seam 
)f chalcedony and kaolin varies from 1 to 2 centimeters. From this 
is a foundation the quartz crystals grew out into the fissure. But 
