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PROCEEDINGS OF THE NATIONAL, MUSEUM. 



VOL. 66. 



a (100), and the base c (001). They are thick tabular, parallel to 

 the base, and are elongated on the h axis, as contrasted with the fore- 

 going, which are elongated on the d axis. The pinacoid a (100) is 

 etched dull but the prismatic faces give good reflections, indicating 

 the angle m (110) a m'" (iTO) = 80° 39'. The basal pinacoid is 

 striated parallel to the h axis as indicated in the drawing. 



Between crossed nicols these crystals also show optical anomalies, 

 the pattern being as shown in figure 18. These are in many respects 

 like the ones previously described but they are somewhat more com- 

 plicated. When the sectors c and c' are at the position of extinc- 

 tion a, a' and h and l' are similarly illuminated and show a uni- 

 form first-order yellow interference color. These sectors give sym- 

 metrical extinction of 8° on either side of 

 the dividing line. The extinction is not 

 uniform, however, but sweeps as a bar 

 from the inner tip of the sector outward. 

 At 45° position all sectors are similarly il- 

 luminated, a and a' , h and b' shade from 

 a broad central yellow field downward 

 through black to blue at the edge, c and 

 c' shade similarly from a yellow central 

 field through black and then have a nar- 

 row outer border of higher colors. No 

 segment is simple. Even the c and c' sec- 

 tors which have homogeneous parallel 

 extinction give, in convergent fight, an 

 interference figure like that obtained from 

 twomuscovite plates superposed at right 

 angles to each other, while the end sectors 

 give still more complicated interference 

 figurs8, suggesting 3 mica plates at 60° to each other. The acute 

 bisectrix of allof the intergrown crystal units is perpendicular to the 

 table. 



The simplest explanation which will fit these several peculiarities 

 is that the crystals are made up, as before, of an underlying homo- 

 geneous crystal which, however, is not of uniform thickness but thick- 

 ens in all directions from the center. Thinning would produce the 

 same result but the thickening is actually noticeable when the prehn- 

 ite crystals are examined. This simple tapering crystal is overlain 

 by layers having the arrangement of the sector pattern, figure 18. 

 In c and c' there is one overlying plate with optical directions at 

 right angles to those of the fundamental crystal beneath. In a — a' 

 and b — h' three layers, the two upper being oriented at 90° to each 

 other and 45° to the underlying crystal. 



/\\^. 



Fig. 20. — Datolite of first genera 

 tion showing acute habit. 



