S. L. Penfield — Cookeite from Maine. 393 



Also ov:i=— :— 



r , r 



r" 

 Similarly 6 u =I- T . 



u 



, The resultant repulsion of the magnetism of the inner sur- 

 face on the unit pole at the center is na / and the attraction of 

 the outer surface is tzg u . Hence, 



Resultant force of magnetic shell = ^(<t / — <t // ) 



=kV (\-^\ =^.^ 



\r r v r 



\' / a I ,i ii 



r 



= 7r I-r( r n+ r ,)( r i- r ,) 



nl 27tl 



r 2 r 



which is the result sought. 



We see too that in the equation 



I=it 



Zis the mean of the two surface densities which must neces- 

 sarily differ in value unless the shell is a plane. 



Aet. XLYI. — On Cookeite from Paris and Hebron, Maine / 

 by Samuel L. Penfield. 



This mineral was first described as a new species in 1866 by 

 Professor George J. Brush,* and only a limited supply of pure 

 material could be obtained at that time for the chemical analy- 

 sis, which was made by Mr. Peter Collier. In the summer of 

 1891 Professor Brush visited the localities of Paris and Hebron 

 and obtained there some excellent specimens of Cookeite, 

 from which abundant material suitable for analysis could be 

 obtained, and at his suggestion the mineral has been reexam- 

 ined. 



Cookeite is related to the chlorites and micas, having a dis- 

 tinct basal cleavage and crystallizing like them in the mono- 

 clinic system. The largest crystals that the author has observed 

 are not over 3 mm in diameter and are hexagonal in habit. Dis- 



* This Journal, II, xli, p. 246. 



