10 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL. 62. 
SUMMARY AND DISCUSSION. 
Of three gouge clays from gold and silver veins in Idaho, which 
were examined, all are homogeneous and crystalline with microscopic 
micaceous structure. All of these, from their occurrence and ap- 
pearance under the microscope, would be classified as sericite. On 
the basis of their alkali content the first would be classified as kaolin 
while the two latter would be interpreted as mixtures of sericite and 
kaolin in approximately equal amounts. On their optical properties 
the first would be considered leverrierite, the second muscovite, and 
the third leverrierite. 
The first described, the clay from the Black Jack vein, is a well- 
defined leverrierite, practically free from alkalies and iron and with 
a large content—nearly 15 per cent—of water which is released at a 
temperature of 110° C. and is quickly regained when the material is 
allowed to stand in air. The second, from the Carroll-Driscoll claim 
in the Boise Basin, Idaho, is a well-defined muscovite optically but 
chemically it is found to be muscovite in which the theoretical potash- 
free hydrogen-muscovite molecule is greatly in excess of the potash- 
muscovite molecule. It contains practically no water which is 
released at a temperature of 110°C. The third clay, from Garfield 
Tunnel, De Lamar district, Idaho, which had been called a mixture 
ef sericite and kaolinite is homogeneous and similar to the last in 
composition but has lower indices of refraction and the equivalent of 
one molecule of water which is released at 110° C. 
From these data it is concluded that muscovite, or at least sericite, 
varies from a theoretical hydrogen-free potash molecule on one hand 
to a potash-free hydrogen molecule on the other and that most seri- 
cites are intermediate. Consequently the potash content is no index 
to the amount of sericite present in a clay and many of those clays 
from veins which have been interpreted, on the basis of their alkali 
content, as mixtures of sericite and kaolinite, may well have been 
homogeneous sericites. Since the presence of sericite does not neces- 
sarily imply the presence of abundant potash it is unsafe to assume 
that this type of alteration in a given area was accomplished through 
the medium of potash-bearing thermal waters until it is definitely 
established by analysis that the sericitized rocks are notably higher 
in alkali content than the same rocks where not affected by the 
alteration. 
Many such sericitic materials are more properly classified as lever- 
rierite although from existing knowledge leverrierite can not be sep- 
arated from sericitic muscovite except on the basis of its low tem- 
perature water. Leverrierite may be defined as a clayey mineral of 
micaceous structure conforming in general with muscovite in com- 
position but with a large content of low temperature water, 1. e., 
