10 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL, 62. 
Although cristobalite is described as twinning frequently on the 
spinel law, no spinel twins were found in the Spokane material. 
The more complex forms of the Spokane cristobalite look so like 
repeatedly twinned tridymite crystals that, were it not for the un- 
avoidable evidence of the optical properties, one might be tempted 
to follow vom Rath’s earlier example and endeavor to interpret them 
as twinned tridymite. Previous to his assigning the name cristo- 
balite vom Rath had described apparently isometric silica crystals 
from New Zealand, which had almost identically the habit of those 
from Spokane. The occurrence is at Littleton Harbor, near Christ- 
church in Canterbury, New Zealand.’® Here a highly crystalline 
erayish-white layer between beds of basaltic andesite contains vesi- 
cles which are invariably lined with a crust of 2 mm. tridymite crys- 
tals, sometimes with some quartz. The writer states that some of 
the crystals in part appear isometric with the cube and octahedron 
in equal development. Occasionally these show skeleton structure, 
and they appear to grade into ordinary aggregates of twinned plates 
of tridymite. The pseudoisometric form is explained as twinning of 
tridymite on r (3034) combined with trilling on g (1016). Itis obvi- 
ous that these cuboctahedral crystals of vom Rath are entirely simi- 
lar to those in the Spokane specimens, although no tridymite has 
been found in the latter. The complicated and improbable expla- 
nation of the origin of the isometric forms is no longer necessary in 
view of the fact that an isometric form of silica has been proven to 
exist. 
It is obvious that in the vesicles of the Spokane basalts the cristo- 
balite, like the plagioclase and magnetite, is a high-temperature min- 
eral deposited during the period immediately following consolidation 
of the inclosing rock. This need not necessarily indicate, however, 
that its formation took place at a temperature above 1,470°, as 
might be inferred from Fenner’s work on dry melts, as abstracted 
above. The presence of volatile constituents in the vesicular cavities, 
during formation of the minerals forming the crust, may have greatly 
altered the physical requirements for their crystallization. 
PYRITE. 
Pyrite was seen to occur sparingly in several specimens and in 
moderate abundance in one. Aside from recording its presence and 
position with reference to the other minerals it does not require 
special description. It is confined to the nonopalized rock and forms 
thin coatings over the feldspar or feldspar-enamel crusts. In age it is 
later apparently than the first generation of sphaerosiderite and earlier | 
than the iron opal. In the specimen showing it most abundantly 
the pyrite coated the gray feldspar glaze completely in places and 
6G. vom Rath. Zeitschrift Karystallographie, vol. 13, p. 599. 
