Il6 MINERALOGY AND LITIIOLOGY. 



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. 



75. Finite. 



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, vv-hich 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. 



76. 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 



