6 PROCEEDINGS OF THE NATIONAL MUSEUM. VoL, 62, 
forms appearing like crystals which had partly fused. The lowest 
index of refraction of the feldspar, a, is 1.5400, and this together 
with its extinction angles identify it as oligoclase-andesine as con- 
trasted with the much more basic bytownite of the enclosing basalt. 
One rather imperfect crystal which was measured on the goniometer 
had the habit shown in figure 2, the forms identified being as shown, 
c (001), 6 (010), a (100), f (180), and 2 (130). Another common form 
is shown in figure 3. While the combinations are simple, the crys- 
tals vary widely in proportions, ranging from slender acicular by 
elongation on axis @ to very broad and thin tabular to the pinacoid 
6 (010). Twins on 6 (010) are common (albite law), and a crystal 
twinned on ¢ (001) was seen (Manebach law). 
MAGNETITE. 
Magnetite is invariably present as minute sharp and _ brilliant 
crystals interspersed with the lath-like crystals of plagioclase in the 
cavities. The crystals are always simple octahedrons, often with 
depressed faces, as shown in figure 
4. The most noteworthy thing 
about the minute magnetite crys- 
tals is their tendency to align 
themselves in strings in parallel 
position, as shown in figure 5. 
CRISTOBALITE.* 
tric form of silica, was first named 
by G. vom Rath* from material 
2. from Cerro de San Cristobal, Hi- 
Figs. 2-3.—COMMON HABITS OF PLAGIOCLASE CRYS- dalgo, Mexico, where it occurs as 
ra octahedral crystals up to 2mm. in 
size and as spinel twins up to 4 mm. in diameter associated with tridy- 
mite, hornblende, and hematite in cavities in augiteandesite. Although 
giving the form a new name, vom Rath himself was inclined to 
believe it to be either pseudomorphous after some isometric mineral, 
such as spinel, or to be merely a form of tridymite which had assumed 
a pseudoisometric form by twinning, Indeed he had previously 
described in detail cuboctahedral forms of ‘‘tridymite’” from New 
Zealand, which, as discussed below, were doubtless cristobalite. The 
same writer also identified as cristobalite a mineral from the lava 
from Ettringen described by Wolf* and by Lehmann’ as druses with 
i 
Cristobalite, the pseudoisome- 
4A preliminary note on the cristobalite has been published. Earl V. Shannon. Journ. Wash. Acad. 
Sci., vol. 12, pp. 195-196, 1922. 
6G. vom Rath, N. Jahrbuch, 1887, pt 1, p. 198. 
6 Th, Wolf, Zeits. der Geol. Gesellsch., 1869, vol. 20, p. 16. 
7J. Lehmann. Ver. Nat.-List. Ver. Rheinl., Bonn, 1874, p. 35. 
