8 PROCEEDINGS OF THE NATIONAL MUSEUM, VOL, 62. 
CLAY GOUGE FROM GARFIELD TUNNEL, DE LAMAR DISTRICT, IDAHO. 
Lindgren ® writes regarding the vein clays of the De Lamar dis- 
trict, Owyhee County, Idaho, as follows: ‘‘ It remains to mention the 
white chalky or ‘talcose’ material often forming parts of the vein. 
This is sometimes a product of metasomatic replacement of the rhyo- 
lite, but may also form the filling between the comb quartz in veins 
and vugs. All of this material is exceedingly fine grained and with 
highest magnifying power appears as scaly aggregates of faint double 
refraction. To judge from the determinations available, it is either 
pure kaolinite or a mixture of that mineral with one of sericitic 
character.’ Lindgren gives determinations of soda, potash, and 
water above and below 100° C. on six gouge clays from various local- 
ities in the De Lamar district and the percentages of these constit- 
uents, while very variable, substantially support his conclusion that 
the clays are mixtures of kaolinite and sericite in various proportions. 
However, while it is true that sericite and kaolinite are both common 
minerals, they represent the results of alteration processes of diver- 
gent character, operating on the same primary materials and it would 
be noteworthy if these processes had acted simultaneously to produce 
a mixture of the two minerals. Some light on the problem is given 
by the analysis of the sericite from Boise County described above 
since that material, though proven by optical methods to be a well 
characterized and homogeneous mica, is of such a composition that, 
on the basis of its water and alkalies contents, it would certainly have 
been interpreted as a mixture of kaolinite and sericite. Only one of 
the original specimens tested for Lindgen was available for more 
thorough examination. This he describes as follows: 
A soft white material from the Garfield Tunnel, south side, end of drift, contain- 
ing much marcasite in arborescent forms yielded: 
Per cent. 
Wateri(Hs0) (= 100%. cot ase ccs apeitaa ia coe cad aes eee 2. 20 
Water GSO) SQ eas oe ata he Se ale ees ht cee een creme 4. 60 
Potassar (Re). cyeae. 25.0 ksi 2 RP SEY oe ee ee 6. 52 
Soda (Nas) cat 2 ag tetcvee co aie tats 255 Rerwals atc See leita .18 
This also is probably kaolinite mixed with a sericitic mineral. 
The modern method of investigating such fine-grained materials 
under the microscope by immersing them in media of known refrac- 
tive index allows the easy detection of mechanical admixtures of min- 
erals of dissimilar indices of refraction. The above-mentioned speci- 
men from Lindgren’s collection was accordingly reexamined with the 
following results. 
Physical properties. —In the hand specimen the material from the 
Garfield Tunnel is a very fine-grained clay. The specimen consists of 
approximately equal parts of marcasite and the clay, the former min- 
6Waldemar Lindgren. Gold and silver veins in Idaho, 20th. Ann. Rept., U.S. Geol. Survey, pt.3, 
pp. 171-172, 1899. 
