234 
ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull.182. 
are si ill recognizable from their outlines, although changed to kaolin 
and diaspore, and finally into a white siliceous rock in which nearly all, 
or quite all, traces of original structure are lost. The exact character 
of this alteration has already been discussed on page 126. The silici- 
fied material of the knob is most irregularly and thoroughly fissured. 
The larger fissures often expand into irregular caves, which, as far as 
could be seen with the limited opportunities for observation in 1899, 
did not run in any single dominant direction. A few of the smaller 
caverns only, near the surface, could be seen. These were lined with 
crumbling, rust-colored, oxidized ore, beneath which there was often 
unaltered galena. Sometimes these caves are lined and partly filled 
with a light, porous, pumiceous sponge of quartz. This is not the 
cellular, honeycombed quartz resulting from the removal of ore by 
oxidizing waters, but. is an original spongy crystallization of minute 
quartz crystals, with some interstitial kaolin. It forms a light friable 
mass that is strongly suggestive of pumice. It is analogous to the 
"sugar quartz" occurring in vugs, with sericite, in some of the gold- 
quartz veins of California, although not to the " sugar quartz " which 
results from a crushing of the vein. 
The National Belle mine has long been noted for the abundance 
and purity of the kaolin found in its workings. 1 It occurs as a soft, 
white filling of fractures in the altered country rock, or in brecciated 
zones as a matrix inclosing fragments of the wall rock. It appeared 
to be, in part at least, of more recent origin than the sulphide ores, 
but no opportunity was available to study its occurrence in the deeper 
workings or in connection with the unoxidized ores. When dry this 
kaolin forms a pure white powder which the microscope shows to con- 
sist of crystal scales, usually hexagonal in outline and about 0.1 mm. 
in average diameter. The optical properties of these crystals have 
been described by Reusch. 2 Two chemical analyses of this material 
are here reproduced, (I) by Hillebrand 3 and (II) by Hiortdahl: 4 
Analyses of kaolinite from National Belle mine. 
Constituent. 
I. 
II. 
SiO- . - 
46. 35 
39. 59 
.11 
13. 93 
.15 
45. 57 
AI0O3 ... 
11.53 
Feo0 3 
HoO 
13. 58 
F 
Less O for F 
100. 13 
.06 
100. 67 
10U.07 
1 R. C. Hills: Am. Jour. Sci., 3d series, Vol. XX VII, 1884, p. 472. 
2 Neues Jahrb. fur Min., etc., 1887, Vol. II, pp. 70-72. 
3 Bull. U. S. Geol. Survey No. 20. 1885, p. 98. 
4 Neues Jahrb. fur Min., 1887, Vol. II, p. 70. 
