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PROCEEDINGS OF THE AMERICAN ACADEMY 



the mass becomes solid, but that subsequently either cleavage may 

 become so dominant as to wholly cover up all other crystalline 

 structure. 



Crystalline Concretions of Mica. 



Some time ago I received from Dr. D. F. Lincoln of Boston some 

 remarkable specimens of curved mica, one of which is shown of natu- 

 ral size in Figure 3. This crystal had its cleavage face very much 



Fig. 3. 



curved, while the successive layers tapered back to an apex at the 

 centre of curvature, the whole having the appearance of forming a 

 portion of a sphere. The occurrence of these micas is very well de- 

 scribed in a letter from Dr. Lincoln : — 



" The specimens of mica were procured by me in 1885, from Mt. 

 Apatite in Auburn, Maine. The rock of that locality resembles that 



