OF CENTRAL CANADA PART II. Ill 



composition, essentially a silicate of alumina and soda, combined 

 with 6 or 7 per cent, of chloride of sodium. 



(8) GROUP OF ZEOLITIC SILICATES. 



[The silicates of this group are essentially hydrous species, especi- 

 ally characteristic of trappean or basaltic rocks. All fuse more or 

 less readily, the fusion in many cases being preceded by intumescence, 

 or accompanied by bubbling, whence the old name of the group from 

 Cew and UOo^. Most of these minerals also gelatinize in acids, or 

 become readily decomposed with separation of granular or slimy 

 silica. Those which occur in Central Canada may be arranged in 

 two sub-groups, comprising (a) Calcareous Zeolites ; and (b) Alkaline 

 Zeolites J\ 



SUB-GROUP A. CALCAREOUS ZEOLITES. 



67. Prehnite : Green, of various shades, greenish-white. Rhom- 

 bic in crystallization, but occurring mostly in botryoidal masses with 

 crystalline surface and radiating fibrous 

 structure (Fig. 71). H = 6.0 6.5 ; sp. 

 gr. 2.8 2.95. BB, easily fusible with 

 great bubbling. Attacked by boiling 

 acids with separation of granular silica, 



FIG. 71. but complete decomposition is not readily 



effected Essential composition : silica 43.5, alumina 25, lime 27, 

 water 4.5. Occurs chiefly in the trap rocks of Lake Superior, some- 

 times forming distinct veins, as on Slate River, an affluent of the 

 Kaministiquia, and with imbedded nodules of native copper on an 

 island near St Ignace. A specimen, obtained by the author from 

 Slate River, shewed a sp. gr. of 2.88, and yielded : silica 43.41, 

 alumina 23.80, sesquioxide iron 1.26, sesquioxide manganese 0.53, 

 lime 26.62, water 4.14. The " Chlorastrolite " from Isle Royale is 

 probably a variety of Prehnite. It occurs in small nodular masses 

 in amygdaloidal trap ; or in the form of small pebbles, left by the 

 disintegration of the trap, on the beaches of the island. Its colour is 

 mostly dark and light green in alternate patches, and it exhibits a 

 radiating fibrous structure and somewhat silky aspect. Sp. gr. 2.90 

 to 3.20, the heavier specimens invariably containing minute particles 

 of magnetic iron ore, forming nuclei from which the fibres radiate. 

 The amount of water varies from a little over 4.0, to about 5.5 per 

 cent. 



