112 MINERALOGY AND LITHOLOGY. 



70. Prehnite [W Ca2 AP Si^ O'-]. 



This mineral is found at Bellows Falls, and also at Franconia. It is a 

 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. Analcite [Na2 AP Si* O'- + 2W 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 PI. 6. The 

 walls of all these cavities were first coated with a yellow, formless, drusy 

 mineral called sphaerosiderite, 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. 



