IYDROMICA GROUP. Sig 
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-schiefer 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, 
and later, by G. F. Barker, for those of Vermont. Puinite 
is related in composition, but is not micaceous. 
Margarodite. 
Like muscovite (page 267), but inelastic. 
Composition. Specimens from the topaz vein, Trumbull, 
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. If 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 eyanite 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 ; Hastern Pennsylvania, ete, 
