496 



PROCEEDINGS OP THE AMERICAN ACADEMY 



process were deposited. Of course, the mere resemblance in the out- 

 line of the two sets of figures is accidental, and arises from the cir- 

 cumstance that the planes of crystalline growth of muscovite mica are 



Fig. 11. Mica, Chandler's Hollow, Delaware. 



parallel to a rhombic prism of sixty and one hundred and twenty 

 degree angles, the right section of such a prism being similar to the 

 section of a regular octahedron parallel to one of its faces. 



The conditions of the plessite which fills the cavities between the 

 crystalline plates of iron meteorites also present features which are 

 especially characteristic of the crystallization of alloys. Sometimes 

 the space is packed with small crystalline plates parallel to those of 

 the external form, the combs already mentioned, shown in Fig. 1. 

 Again the material is granular, as shown at a, b, and c, Fig. 2, or 

 again divided into polygonal masses as shown in the same figure. 

 Similar features in the alloys of zinc and antimony have been de- 

 scribed by Professor Cooke,* and in the alloys of copper and zinc 

 by Prof. F, H. Storer.f They correspond to that pasty condition 



* Memoirs of the American Academy, New Series, vol. v. pp. 336-371. 

 t Ibid., vol. viii. pp. 27-56. 



