PREHNITE PROM ADAMS SOUND 
97 
distinct prismatic cleavage. Prof. Pirsson, who has examined 
the mineral optically, has furnished the following notes regarding 
it: birefringence >0*20; index of refraction >0*16; parallel 
extinction; optic axial angle positive. The colour is a very pale 
green; it is perfectly translucent and has a weak, pearly lustre. 
In thin sections under the microscope the prehnite is seen in 
the form of sheaves and bundles of minute crystals mixed with 
more or less of the calcite and some quartz. Its hardness is 
about 6. Before the blowpipe it fuses with swelling and 
contortion at about 3 * 5 to a brownish slag which is not easily 
further fused. On strong ignition in a closed tube it yields 
water. Previous to fusion the finely divided powder is not 
readily acted upon by strong hydrochloric acid; after fusion it 
gelatinizes perfectly. 
For the purposes of an analysis a number of pieces of the 
mineral were first freed as far as possible by hand from asso- 
ciated quartz and calcite and then crushed to pass through a 
sieve of twenty meshes to the linear inch; the crushed material 
was then treated with dilute hydrochloric acid (1 HC: 6 H 2 0) 
to remove calcite; it was then washed and allowed to dry at 
the ordinary temperature of the air for twenty-four hours, after 
which it was introduced into a Penfield’s separatory tube 
charged with Thoulet solution of 2*8 S.G. The heavy separate 
was after a time removed, washed with ET solution and after- 
wards with pure distilled water; it was then spread out on glass 
plates and allowed to dry. As will be seen subsequently, 
however, this treatment probably did not effect a complete 
separation of the quartz, as the analysis shows an excess of 
4*38% of silica over that required for normal prehnite. Its 
specific gravity at 15 ♦ 5°C. was found to be 2 * 924, and an analysis 
of the material prepared as indicated above gave the following 
figures : — 
Silica (SiO*) 44-35 
Alumina (AljOa) 19*44 
Ferric oxide (FeaOa) 6*58 
Calcium monoxide (CaO) * . 25*50 
Water (HaO) 4 -Of) 
99*87 
Chlorine and fluorin were sought quantitatively by the 
method of Berzelius, but with negative results in each case. 
