116 MINERALOGY AND LITHOLOGY. 
erable interest. What the mineral is now cannot be said with certainty, 
since it is not homogeneous. It has at least got quite near in composi- 
tion to some of the kinds of clays. It occurs chiefly in cavities of the 
slaty rocks. 
45. PINITE. 
Under the head of pinite, a number of amorphous products are classed 
together by Prof. Dana, which are essentially hydrous silicates of alumina 
and potash, and which have resulted from the decomposition of alkaline 
silicates. A green substance, which has been referred to pinite, occurs 
in the granites of Bellows Falls, and in the protogene rocks at Littleton 
and other places in the neighborhood. These substances, when examined 
in thin sections under the microscope, resemble serpentine, a mineral to 
which, in its mode of origin, pinite is closely related. 
46. MARGARODITE—SERICITE. 
Muscovite, as is well known, is exceedingly subject to hydration, and 
while yet maintaining its physical and optical properties, gives, on analy- 
sis, considerable water. This change is evinced in an increase of pearly 
lustre and opacity, but in other characters it is still like muscovite. A 
large amount of the mica in our rocks is more or less hydrous, and may 
be called margarodite, if one so choose; but in the study of lithology 
there is a stage beyond this, where a hydrous mineral, nearly related to 
these in composition, has none of the characters of muscovite, and is a 
fibrous mineral resembling talc, from which it is distinguished by its 
composition. This mineral was called sericite by List, on account of its 
silky lustre; and the rocks containing it have been called sericite schist, 
gneiss, etc. Rosenbusch believes that sericite is a well established spe- 
cies, thoroughly distinguished from micas by its fibrous structure, while 
Lasaulx regards this subject as needing investigation, and thinks that 
various micaceous minerals are included under the name sericite. Prof. 
Dana calls these rocks, which contain this soapy, talc-like mineral, hydro- 
mica schist; and although, while the question ‘stands as to-day, 'this is 
the best name, yet it may be stated that we ‘have in our ‘rocks, first, a 
hydrous mica with a micaceous structure, and with the optical properties 
of muscovite. It is usually yellow in thin sections, and shows the cleav- 
age very distinctly. Again: we have in other rocks a fibrous mineral 
