112 MINERALOGY AND LITHOLOGY. 
40. PREHNITE [H? Ca? Al? Si? OF]. 
This mineral is found at Bellows Falls, and also at Franconia. It isa 
green mineral, orthorhombic in form, which results from the alteration 
and decomposition of other minerals, especially of the basic minerals of 
trap rocks. It is inconspicuous in our state. It is found only in thin 
crusts, which appear almost amorphous, being aggregates of so many 
small crystals. 
71. AnatciTE [Na’ Al? Sit O8 +. 2H? O}. 
I am aware of only one occurrence of this mineral, and that a micro- 
scopic one, which presents the usual optical peculiarities of this species. 
Some of the augite porphyry at Campton falls is filled with little micro- 
scopic cavities. These cavities are represented in Fig. 3 on Pl.6. The 
walls of all these cavities were first coated with a yellow, formless, drusy 
mineral called sphzerosiderite, a carbonate of lime and iron. Then there 
was a growth of hexagonal calcite crystals, terminated in some cases by 
the planes of an obtuse rhombohedron, and sometimes extending entirely 
across the cavity; and, lastly, the remaining room in the cavities was 
entirely filled with analcite. This analcite shows a quite fine cubic cleav- 
age, a thing not often macroscopically seen in such perfection. Analcite 
is isometric. Isometric crystals are black between crossed Nicols, and 
act like amorphous bodies, but this analcite does not so behave. Some 
few of its sections are black; but the larger part are dark only in certain 
positions, and on revolving them they assume a bluish-black color, be- 
come sensibly lighter in shade, and become black again when they have 
been revolved 90°, and thus, though faintly, they show all the peculiar- 
ities of prismatic crystals. This deportment has caused serious doubts 
to be thrown upon the isometric character of analcite; and leucite, 
which crystallizes like analcite, and shows the same peculiarities in a 
somewhat more marked degree, is quite satisfactorily proved to be tetrag- 
onal. The anomalies of analcite were first noticed by Brewster; and 
Des Cloizeaux subsequently determined that sections cut parallel to any 
of the cubic faces act in parallel polarized light like isometric crystals,— 
that is, light passing parallel to any axis is not modified. Analcite, 
which like this entirely fills cavities, has been noticed in several basaltic 
rocks, chiefly Italian. 
