MINERALOGY, 117 
which bears no resemblance to mica, which, however, is certainly related 
to it in composition, which looks in the microscope like talc, and gives 
its character to the rocks, and is what has been called sericite. Such 
rocks occur at Northumberland, and at various points on the Connecticut. 
Lasaulx shows this mineral to be a product of the decomposition of feld- 
spar in some cases. 
In regard to margarodite, it may be noted that a visit to our mica 
mines shows how quickly muscovite is turned to margarodite, after a lit- 
tle exposure. The mica that is rejected, and thrown into piles on one 
side, very quickly becomes hydrous, loses its transparency, and becomes 
silvery. A piece exposed less than a year gave me 4.2 per cent. of water ; 
and hence we might expect that the mica, wherever exposed, would be 
hydrous. 
A hydrous mica is found at Enfield, associated with quartz, which 
forms rounded mammillary forms, composed of excessively minute scales. 
The whole appearance of the mica resembles prehnite, but it is nearly 
infusible, and close examination reveals its micaceous structure. I 
have examined this mica, and have found it to be a soda-potash mica, 
intermediate between margarodite and paragonite. It resembles one 
analyzed by Smith and Brush, from Litchfield, Conn.* The specimens 
examined were furnished by Mr. Downs. These intermediate species 
between the hydrous, potash, and soda micas, indicate that the dividing 
lines between them are indefinite. 
CHLORITE. 
Under this head it is proposed to describe those minerals which, 
though having essential chemical differences, yet have those well known 
‘properties in common that cause them to be all usually-called chlorite. 
The chlorites are hydrous silicates of magnesia, iron protoxide, and 
alumina; their hardness varies between that of talc and gypsum; they 
are foliated like mica, but their folize are not elastic as are those of mica; 
and they are of various shades of green, according to the amount of iron 
which they contain. The three most common species of chlorite are the 
monoclinic ripidolite, the rhombohedral -penninite, and the hexagonal 
prochlorite. The first is biaxial, the second and third uniaxial, though 
* See “‘ Margarodite,’”” Dana’s Mineralogy. 
