of several American Micas. 67 



mens. No analysis of it has been published, but the author 

 hopes to present one at a future time. 



The white mica of Easton Pa., which is very silvery and 

 slightly elastic and opaque in thick plates, is probably abiotite, 

 and, excepting the very similar white mica of Amity, N. Y., 

 is the only white uniaxial variety yet noticed in this country. 



The Black Micas are almost universally referable to the 

 species biotite, although many micas usually called black in 

 collections are in reality dark brown and olive green, and are 

 frequently referable to phlogopite. Unfortunately very few 

 of the localities of this variety of colour found in cabinets are 

 labelled. I have one from Moors Slide on the Ottawa in 

 Canada, furnished me by Mr Hunt of the Canada Geological 

 Commission. Another black mica is found in St Lawrence 

 Co., N. Y., of which specimens were obtained by the author 

 from the cabinets of Mr Wheatley and of the N. Y. Lyceum 

 in New York. Two black micas from the Cambridge cabinet 

 are uniaxial, locality not known. 



Geological Belations. — It is worthy of notice that the species 

 muscovite is found almost entirely in granitic rocks ; in no 

 instance as far as I have seen, has a specimen of this species 

 been found in a lime rock. On the other hand, the phlogo- 

 pites, with a single exception, so far as has been ascertained, 

 are found in limestone and often in dolomitic beds. The 

 biotite is less well determined, but, as regards the black micas, 

 they are always, it is believed, found in granitic rocks. Thus 

 New England is the region of muscovites, and northern New 

 York, New Jersey and Canada, that of phlogopite. The few 

 specimens of muscovite from St Lawrence Co. were obtained 

 from granite boulders. The only specimen of phlogopite yet 

 observed in New England is from Carlisle in Mass., where 

 also it is found in a limestone vein with chondrodite and 

 fibrolite, an interesting confirmation of the suggestion here 

 put forth. Can this distribution be unconnected with the 

 chemical composition of the several compounds ? The mag- 

 nesian character of the phlogopites would seem to indicate 

 the dolomitic position of the species, while the absence of this 

 element in the muscovites is a negative fact of equal signifi- 

 cance. 



