366 Haworth on the Archcean Geology of Missouri. 
montite is in small, irregular grains which, while not being in 
well-developed crystals, are often elongated sufficiently to show 
their axial directions. 
In order to determine whether or not the mineral was pied- 
montite it was subjected to the following examinations. A 
careful microscopic examination was made, which showed that 
a portion, at least, of the grains extinguished parallel to their 
greatest elongation. Piedmontite is elongated parallel to crys- 
tallographic b, and all sections in the zone of the basal-pinacoid 
and orthopinacoid planes would have parallel extinction. The 
mineral is strongly pleochroic, giving d = carmine, l* = ame- 
thyst, t = orange. These correspond quite well with the colors 
given by Laspeyres' for piedmontite from St. Marcel. The 
index of refraction is quite strong, as is shown by the apparent 
thickness of the mineral in the slide. A quantity of the mineral 
was isolated by first making a separation of the portions of the 
rock heavier than 3.1 by means of the Thoulet solution, and 
boiling the powder thus obtained in strong chlorhydric acid, 
which dissolved the iron oxides, leaving the piedmontite mixed 
with epidote. During the boiling examinations were made to 
see if free chlorine were given off, which would have been the 
case had manganese dioxide been present. None whatever 
could be detected. The residue looked quite red, and gave a 
strong reaction for manganese. 
Now the only two minerals with which piedmontite is lia- 
ble to be confused are withamite and thulite, two varieties of 
epidote. Rosenbusch, in the last edition of his petrography, is 
inclined to class with withamite all the red epidotes which he 
inentions as occurring in porphyries — see p. 472. Lacroix" has 
investigated the optical properties of withamite and thulite, and 
finds that they correspond very closely with those given b}' 
Laspeyres for piedmontite. Thus, all three are positive; the 
three pleochroic colors are the same for the three minerals, and 
have the same arrangement in piedmontite and withamite, but 
different in thulite. The colors in piedmontite, however, are a 
little more strong than in either of the others. 
The different text-books examined which mention withamite 
1 Zeitschr, fiir Kryst, vol. 4, pp. 435-468. 
- Bull, de la Soc. Francaise de Min., p. 75. 
