298 DESCRIPTIONS OF MINERALS. 
sures in which the minerals occur; and part were made 
while the rock was still hot, and as cooling went forward. 
Besides true zeolites, such cavities often contain also 
Laumontite (p. 293), noted for its tendency to crumble 
on exposure; Pectolite and Okenite (p. 293), which are 
fibrous like Natrolite and Scolecite ; Apophyllite (p. 294), 
having one pearly cleavage like heulandite and stilbite ; 
Prehnite (p. 295), usually apple-green ; Datolite (p. 289), 
in stoutish glassy complex crystals, or in smooth botryoidal 
forms ; Aragonite (p. 218), sometimes radiated fibrous, and 
Calcite (p. 215) with its three directions of like easy cleay- 
age, both effervescing with hydrochloric acid; Siderite 
(p. 185), in spheroidal or other forms; Chlorite (p. 316), 
of dark olive-green color ; and Quartz, either in crystals, 
or as chalcedony, agate, or carnelian, and in either case 
easily distinguished by the hardness, absence of cleavage, 
and infusibility. Of all these species Calcite and Quartz 
are the most common. Of rarer occurrence than the above, 
there are Orthoclase, Asphaltic coal, Copper, ete. 
All the zeolites yield water in the closed tube, and many 
of them gelatinize with hydrochloric acid. 
Thomsonite. 
Trimetric. In right rectangular prisms. Usually in 
masses having a radiated structure within, and consisting 
of long fibres, or acicular crystals ; also amorphous. 
Color snow- white ; impure varieties brown. Lustre vi- 
treous, inclining to pearly. Transparent to translucent. 
H.=5-5. Brittle. G.=2:3-2-4. 
Composition. (Ca,Na,)Al Os Si,+24 aq=Silica 38:09, al- 
umina 31°62, lime 12°60, soda 4°62, water 13°40=100°20. 
B. B. fuses very easily to a white enamel. Decomposed by 
hydrochloric acid; the solution gelatinizes when evaporated. 
Diff. Distinguished from natrolite by its fusion to an 
opaque and not toa glassy globule. 
Obs. Occurs in amygdaloid, near Kilpatrick, Scotland ; 
in lavas at Vesuvius, Co mptonite; in clinkstone in Bohemia; 
the Tyrol, etc.; at Peter’s Point, Nova Scotia, in trap; a 
massive variety, called Ozarkite, at Magnet Cove, Ark. 

