184 PERIDOTITE. 



of silica and other impurities. We further find various gi'adations in 

 different specimens, and even partly in the same grain, between the yel- 

 lowish- to reddish-brown mineral, and that which is opaque and crystalline- 

 granular, with or without magnetite or some of the other iron ores, like 

 limonite or hematite. 



The above investigations apparently point to the following conclu- 

 sions : — 



That picotite in its freshest state is, in the thin sections, a yellowish- or 

 greenish-brown, clear mineral which is subject to alterations, causing the 

 color to deepen to a darker brown or muddy coffee-color, and even to a 

 black and opaque mass. The changed forms vary from a dull, earthy mass 

 to a crystalline-granular one which frequently contains more or less magne- 

 tite. Commonly, the more the rock is altered or changed to a serpentine, 

 the less apt is one to find any of the translucent grains, and the greater is 

 the amount of magnetite. 



It is probable that picotite and chromite belong to the same mineral 

 series, the term picotite being more commonly applied to the freshest states, 

 and that of chromite to those forms more altered, and to the local aggi'cga- 

 tions arising from the migration of the chromic oxide during the alteration 

 of the associated peridotic rock. 



The results deduced from the microscopic study of minerals, lead to the 

 conclusion that most of our mineral species are not definite compounds 

 corresponding to a special chemical formula, but rather are varying and 

 variable compounds. They are seen to contain inclusions of various kinds, 

 and to exist in various stages of alteration and decomposition. This we see 

 in the case of micaceous products which occur in certain greenish, fibrous, 

 and scaly forms, produced from the alteration of various minerals. Now, 

 while these forms are microscopically undistinguishable from one another in 

 their earlier stages, they are seen to result in their further change, in the 

 production of biotite, hornblende, chlorite, hydrous micas, etc. Now shall we 

 give a distinct name to each of these variable aiid interminable micaceous 

 forms, whenever from its analysis we can torture a chemical formula into 

 being ; or shall we recognize the variability, and use our mineral names 

 simply as type-names about which the related products in their various 

 stages are grouped ? The latter is the method emplo^'ed in this work. The 

 variability of mineral species not only occurs in the micaceous minerals above 

 mentioned, but it is seen in picotite and the ores of iron, the feldspars, serpen- 



