ART. 15, MINERALOGY OP GOUGE CLAYS—-SHANNON. 9 
eral having a peculiar dendritic form from the arrangement of its 
crystals in branching fernlike growths. Although interiorly the clay 
is white as mentioned by Lindgren, the exterior of the specimen is 
now pinkish and fresh fractures gradually turn from white to greenish 
and finally pink, probably from the oxidation of minute amounts of 
ferrous sulphate formed by breaking down of the marcasite. The 
clay portion of the specimen is compact and shows contraction cracks. 
When soaked in water the material cracks up gradually without con- 
spicuous swelling. 
Optical properties.—Under the microscope the material is very fine 
grained and is composed of aggregates, shreds, and flakes which 
give a brown dispersion color in transmitted light. Between crossed 
nicols the mineral shows rather high birefringence. It is seen, when 
immersed in an oil of its own refractive index, to be homogeneous 
although slightly variable in index, with the following average indices: 
a=1.547, vy =1.556, y—a=0.009. 
The apparent birefringence is much greater, as seen under the micro- 
scope, then the birefringence indicated above. Upon standing in 
oils for about a week the indices had not changed appreciably. 
Composition and chemical properties.—Upon analysis selected mate- 
rial from this specimen yielded the following results: 
Analysis and ratios of clay from Garfield Tunnel. 
Sh Osea HAR Se ereless 6 45, 94 0. 7618 76.18 15. 245 1.005 
ALO se wee ee Se 34. 14 p tae 
2 $4.20 17.10X2 1.122 
Rant t een 126” «0079 mn “ss 
CAOU ALE Sk SPR EMED. . 84 . 0149 
Moon. ses! bas 18 —-.0044 
Ki Oves sbrond oerg- re 6.52 0692 47.97 15.993 1.053 
War (Ones 6) nse Sige t. 0027 
HEOARTI02 6. o..<a5 7. 00 . 3885 
lal (Qe LI) Cee sth) . 1500 15. 00 15.001 1.001 
Motaliee tesa 98. 76 
Chemically this, like the sericite from Boise County described 
above, is like a muscovite in which the potash has to a considerable 
extent been replaced by water although ratios of this last analysis do 
not agree quite so well with the simplified muscovite formula, the 
higher silica ratio indicating some admixture of the phengite mole- 
cule. The bases also are high as compared with the silica but, owing 
to the finer and more felted structure, this sample probably has more 
adsorbed water than the Boise County specimen. The chief chemi- 
cal difference, however, lies in the additional molecule of water which 
is given off at 110°C. On the basis of this water content and its in- 
dices of refraction this sample might be called leverrierite although 
this content of water is much less than that ordinarily found in lever- 
rierite. 
60466—23—Proc.N.M.vol.62 
36 
