64 



PEOCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 66. 



Fig. 14.— Prehnite. Type i 



SHOWING A MODIFICATION 

 OF THE "hour-glass" 



structure where the 

 two end sectors are not 

 connected. 



When this main portion of the crystal is at the position of extinc- 

 tion the end portions, a — a^ and h — h\ have a uniform and 

 identical first-order yellow birefringence color, which is probably 

 their maximum birefringence. Their optical directions are thus 

 inclined 45° to those of the main portion of the 

 crystal. They are moreover inclined 90° to 

 each other, as is shown by the insertion of a 

 first-order red gypsum plate when the sectors 

 a — a^ become blue and h — b^ become yellow 

 or vice versa. 



These end sectors a — a ' and h — h^ yield no 

 true extinction between crossed nicols, but only 

 sweeping bars as the stage is rotated, and their 

 birefringence colors are 

 abnormal low-order blue 

 and liver brown. Any 

 area in these sectors, in 

 convergent light, gives a 

 confused interference fig- 

 ure which is biaxial, positive, with 2V very 

 small to small, r<v extreme, crossed disper- 

 sion extreme, acute bisectrix normal to the 

 plate. 



The example illustrated in figure 14 is the 

 same case except in the 

 shape of the "hour-glass" 

 pattern. 



In the case of these crystals, which are the 

 simplest examined, the anomalous optical be- 

 havior can most easily and satisfactorily be ex- 

 plained by assuming an underlying normal preh- 

 nite crystal, of uniform thickness and normal 

 optical properties, overlain by a scale having 

 the outline of the hourglass and made up of 

 two crystal individuals oriented at right an- 

 gles to each other and at 45° to the underlying 

 crystal. All of the anomalous birefringence, 

 dispersion, and confused optical figures can be 

 simply accounted for by this interpretation. 



The crystal illustrated in figure 15 is similar. 

 Here the plate is rectangular and is bounded 

 only by the pinacoids a (100) and h (010). When the main crystal 

 c — c' is at the position of extinction, which is parallel to its edges, 

 the hour-glass portions show only a very faint luminosity, which 

 gives the same effect as the preceding with the sensitive tint, but 



Fig. 15.— Prehnite. Type i 

 showing hour-glass struc- 

 ture IN A crystal bound- 

 ed only by pinacoids. 



^---N 



Fig. 16.— Prehnite. A vari- 

 ant OF type 1 showing the 



G R O W T H OF thickening 



at the ends to produce 

 sheaves. 



