HYDROMICA GROUP. 313 



covite, readily becomes hydrated on exposure ; but hydrous 

 micas are not all a result of alteration. The Hydromica 

 slates form extensive rock-formations, equal to those of the 

 ordinary mica schists. They were for the most part called 

 Talcose slates (or Talk-scMefer in German) from their greasy 

 feel, until the fact was ascertained that they contained no 

 magnesia : a point demonstrated for the Taconic slates of 

 the western border of Massachusetts, by C. Dewey, in 1819, 

 aud later, by G. F. Barker, for those of Vermont. Pinite 

 is related in composition, but is not micaceous. 



Margarodite. 



Like muscovite (page 267), but inelastic. 



Composition. Specimens from the topaz vein, Trtimtmfl, 

 Conn., afforded Silica 46-50, alumina 33*91, iron sesquiox- 

 ide 2*69, magnesia 0*90, soda 2 -70, potash 7*32, water 4*63, 

 fluorine 0-82, chlorine 0*31 = 99*78. Another from Litch- 

 field, Conn., accompanying cyanite, afforded water 5*26 per 

 cent., soda 4*10, potash 6*20, showing a large percentage of 

 soda. It is probable that both of these micas were originally 

 hydrous. 



Damourite. 



Mica-like, consisting of an aggregation of fine pearly scales, 

 yellow to white in color. 



Composition. Near margarodite, being a hydrous potash 

 mica. A specimen from Brittany afforded Silica 45*22, 

 alumina 37 "85, potash 11 -20, water 5*25 = 99*52. The quan- 

 tivalent ratio for the protoxide, sesquioxide, silica, and 

 water is 1: 9: 12:2, instead of that of margarodite, which is 

 1:6:9:2. A schistose hydromica slate from Lehigh County, 

 Pennsylvania, afforded Dr. Genth, Silica 49*92, alumina 

 34*06, iron sesquioxide 0*91, magnesia 1*77, lime 0*11, soda 

 0*74, potash 6*94, water 6*52 = 100*97. 



Obs. From a locality of cyanite in Brittany, and another 

 in Warmland ; also the constituents of a garnetiferous schist 

 at Salm-Chateau, in Belgium; and in part of extensive 

 schistose formations in Vermont, Western Massachusetts, 

 Western Connecticut, and also just west of New Haven, 

 Connecticut ; Eastern Pennsylvania, etc. 



