82 MINERALOGY AND LITHOLOGY. 



highly ferruginous minerals, when the iron is present in the state of 

 protoxide, are very subject to oxidation. A number of such micas have 

 been analyzed, and they show no essential difference from such a biotite 

 as that last described, save that they contain a large proportion of iron 

 sesquioxide, and are black and opaque, and more or less brittle. The 

 optical properties, when they can be made out, are identical with those of 

 biotite. These micas, containing sesquioxide of iron in the place of 

 protoxide, are called lepidomelane. Many specimens of mica which 

 are found in the granite quarries and rocks of New Hampshire would 

 be referred to this species. It is distinguished from pure biotite by its 

 opacity, its lack of elasticity, and its black shiny lustre. It fuses before 

 the blow-pipe to a magnetic globule, and is easily decomposed by acids. 

 In the microscope it has the same optical properties as biotite, and it is 

 common in some of our granitic rocks. Prof. J. P. Cooke* describes 

 a variety from Cape Ann, which has been named annite, which differs 

 from what is considered typical lepidomelane in containing a less pro- 

 portion of iron sesquioxide, and a greater of protoxide. We have, in some 

 of our granites, micas which exactly answer to this in physical proper- 

 ties ; a granite from Farmington contains mica like this, which gives 

 the reaction for lithia that is obtained from annite. Prof. Cooke, thinking 

 this to be owing to an impurity, deducts the percentage of lithia ob- 

 tained ; but as my analysis shows that lithia exists in pure biotite, I 

 think that our micas that I have examined, and which give a lithia reac- 

 tion, doubtless contain it in their composition. 



The optical properties of lepidomelane under the microscope are 

 identical with those of biotite. It is quite likely that a large proportion of 

 the black mica in our rocks might be referred to this species ; but in 

 lithology no distinction between lepidomelane and biotite is recognized, 

 and all the black dichroic micas are called biotite. 



56. Muscovite [K^ AP Si^ 0«]. 



Muscovite, or common mica, is one of our most valuable minerals. 

 Besides its universal distribution over the state as a rock constituent, 

 large amounts of it are extracted from the granitic veins, where valuable 

 plates, sometimes a yard across, are found. Grafton, Alstead, Acworth, 



* Am. your. Science, u, vol. xliii, p. 22. 



