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Penfield and Pratt — Chemical 



the appearance is that of a simple dark cross, fig. 4 ; while 

 intermediate sections show the rhomb diminishing in size as 

 the sections approach the middle of the crystal, figs. 2 and 3. 

 A number of crystals were cut showing this same phenomena 

 and the symmetrical arrangement of the rhomb and cross was 

 always well marked. The central portions a and the outer ones 

 b, up to the very edges of the section, are remarkably pure 

 staurolite. The dark bars running parallel to the macro axis 

 broaden as they approach the outer angle of the section and 

 are more regular and better defined than the brachy-diagonal 

 ones. From a series of sections, then, it is evident that each 

 staurolite prism contains two skeleton or phantom pyramids P, 

 outlined by carbonaceous material, whose bases correspond to 

 the basal planes of the staurolite and whose apices join at the 

 center, while from the acute and obtuse pole edges of the 



pyramids the inclusions extend as films or fins A and B to the 

 vertical edges of the prism, fig. 5, the numbers at the side of 

 the figure indicating where sections should be cut to give the 

 phenomena corresponding to figures L to 4 respectively. Regu- 

 larly arranged inclusions have previously been observed in 

 staurolite* but apparently they have never been studied from 

 a series of sections from a single crystal. 



In seeking for an explanation of these inclusions, it must be 

 borne in mind that staurolite is a mineral occurring essentially 

 in the crystalline schists, which were probably derived from 

 former mud or clay deposits. The crystals were formed by 

 metamorphic agencies, under great pressure, in rocks which 



* C. T. Jackson, Alger's Phillips Mineralogj^, 1844, p. 112 ; Dana's Min., Sixth 

 edition, p. 560 : S. Webber, Proc. Nat. Institution for the Promotion of Sci., Bull. 

 2, p. 197, 1842 ; A. Lacroix, Min. de la France, 1893, p. 11. 



