HYDROUS SILICATES — GENERAL SECTION. 295 



Dif. The pearly basal cleavage and the forms of its glassy 

 crystals at once distinguish it from the preceding species. 

 The prisms are sometimes almost cubes, with the angles 

 cut on 2 by the planes of the pyramid ; but the difference in 

 the lustre of the prismatic and basai faces shows that it 

 is dimetric. 



The name alludes to its exfoliation before the blowpipe. 



Obs. Found in amygdaloidal trap and basalt. 



Occurs in fine crystallizations at Peters Point and Par- 

 tridge Island, Xova Scotia, at Bergen Hill, N. J., the Cliff 

 Mine, Lake Superior region. 



Catapleiite. A hydrous zirconium and sodium silicate, from Nor- 

 way. 



jDioptase and Chrysocolla. Hydrous copper silicates. See p. 141. 



Pkrosmine, Pyrallolite, Picrophyll, Traversellite, Pitkarandite, Stra- 

 konitzite, Monradite, are names of varieties of pyroxene in different 

 stages of alteration. Xylotine is probably altered asbestus. 



Prehnite. 



Trimetric. IaI=09° 50'. Cleavage basal. Sometimes 

 in six-sided prisms, rounded so as to be barrel-shaped, and 

 composed of a series of united plates ; also in thin rhom- 

 bic or hexagonal plates. Usually reniform and botryoidal, 

 with a crystalline surface ; texture compact. 



Color light green to colorless. Lustre vitreous, except 

 the face 0, which is somewhat pearly. Subtransparent to 

 translucent. H. = G-G'5. G. =2-8-2 "96. 



Composition. H 2 Ca 2 A10i 2 Si 3 =Silica 43*6, alumina 24*9, 

 lime 27*1, water 4*4=100. B.B. fuses very easily to an en- 

 amel-like glass. Decomposed by hydrochloric acid, leaving 

 a residue of silica in light flakes, but the solution does not 

 gelatinize. Yields a little water when heated in a closed 

 tube. 



I) iff. Distinguished from beryl, green quartz, and chal- 

 cedony by fusing before the blowpipe, and from the zeolites 

 by its superior hardness. 



Obs. Found in the cavities of trap, gneiss, and granite. 



Occurs in the trap rocks of the Connecticut Valley, and 

 at Paterson and Bergen Hill, N. J. ; in gneiss at Bellows 

 Falls, Yt. ; in syenyte at Charlestown, Mass. ; and very 

 abundant, forming large veins, in the Copper region of Lake 

 Superior, three miles south of Cat Harbor, and elsewhere. 



The Fassa Valley in the Tyrol, St. Christophe in Dau- 



