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 Xatrolite 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 cleav- 

 age, both effervescing with hydrochloric acid ; Siderite 

 (p. 1S5), 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, etc. 



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 pearlv. Transparent to translucent. 

 H. =5-5. Brittle. G. =2-3-2-4. 



Composition. (Ca.XaJAl O s Si 3 + 2J aq=Silica 38*09, al- 

 umina 31-02. 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 to a glassy globule. 



Obs. Occurs in amygdaloid, near Kilpatrick, Scotland ; 

 in lavas at Vesuvius. Comptoniie; in clinkstone in Bohemia; 

 the Tyrol, etc. ; at Peter's Point, Xova Scotia, in trap ; a 

 massive variety, called Ozarkite, at Magnet Cove, Ark. 



