and Shepard's Treatise on Mi7ieralogy. 345 



hesitate to place the stellite of Dr. Thomson, containing six per 

 cent, of water, with pectoHte, beheved, as Mr. Alger states, to be 

 anhydrous. He also suggests that the mineral is an anhydrous 

 lime-mesolite, an unfortunate name, as it creates confusion to 

 extend the name mesolite to an anhydrous mineral ; for on no 

 principle yet discovered, can the one (hydrous) be isomorphous 

 with the other. The crystals of Bergen Hill, examined by Mr. 

 Teschemacher, are stated by Mr. Alger to have had the form of 

 mesolite, and on this ground the name is proposed. Frankenheim 

 suggests that pectolite is a tremolite with part of the lime replaced 

 by soda. Still farther examination is required before the identity 

 of the Bergen Hill mineral with pectolite can be considered as 

 established. 



The important subject of polarization of hght is very briefly 

 treated by Mr. Alger, and is scarcely mentioned by Dana and 

 Shepard. It may be said, that as a branch of optics it has only 

 remote relations to mineralogy, and ought to be reserved for the 

 class room of the professor of natural philosophy. To say nothing 

 of the elegant and attractive nature of the subject, and the ease 

 with which some of the more striking phenomena are made con- 

 spicuous to a class of mineralogical students, we cannot deny 

 that it has a most important bearing on the determination of min- 

 eralogical species. By its means we often derive important aid in 

 determining the system of crsytallization to which a mineral be- 

 longs, and in distinguishing species that otherwise might be 

 thought identical. 



A single instance may serve to illustrate our remarks. The spe- 

 cies mica, as is well known, has been divided into hexagonal and 

 oblique or common mica, belonging to different systems of crys- 

 tallization. This division, which was based on crystallographic 

 grounds, has been confirmed by the optical properties of the two 

 minerals. But further, the hexagonal mica of Henderson, N. Y. 

 has been found to have the optical relations of binaxial mica, and 

 therefore, instead of having a hexagonal primary like other hexa- 

 gonal mica, its primary is a right rhombic prism, and it is hence 

 entitled to rank as a distinct species.* 



* Mr. Alger has so placed it as magncsian mica, from the large proportion of this 

 earth found in it by Meitzendorf, (Pogg. lviii, 157, 1843.) The same name has 

 been given to hexagonal mica, as magnesia is its characteristic ingredient. Dana 

 proposes the less objectionable name of rhombic mica. 



