16 PROCEEDINGS OF THE NATIONAL MUSEUM vol.73 



a=1.488, /3=1.498, y=1.500; birefringence^ 0.012. The analysis gave 

 the following results : 



Analysis and ratios of stilMte 



Constituent : Per cent Ratios 



SiOa 56.24 0.933 0.933 0.155X6 



AI2O3 17.16 .168 .168X1 



CaO 8.56 .153] 



MgO .40 .OlOl Tp„ i«RN/i 



K2O .32 .003 -1^6 .166X1 



NazO Trace. J 



H2O-l-120(7C 16.80 .9331 qq„ i«avr 



H2O-120° C .96 .053/ ' ^^^ .164X6 



100. 44 

 The composition is that of an ordinary stilbite represented by the 

 formula : 



CaO.Al203.6SiO,.6H,0. 



Another group of stilbite-bearing specimens is that containing 

 mesolite. The stilbite occurs as white crystals up to 1 by 3 by 5 

 millimeters, entangled in the loose fibers of the pseudomesolite or as 

 loose aggregates occupying the center of the cavity. The stilbite is 

 the youngest mineral in these specimens. The crystals have the 

 usual form — tabular parallel to 5(010), with pearly luster on the 

 broad pinacoid, and terminated by a rude pyramid. They are aggre- 

 gates, tending toward a sheaf form, of small individuals not quite 

 in parallel position. Some small specimens show these irregular 

 crystal aggregates, or sheaf-like crystals, alone filling narrow seams 

 up to 1 centimeter wide. Another specimen shows them resting upon 

 a thin crust of white rhombohedral crystals of chabazite and in 

 another they rest upon a thin druse of tiny heulandites. 



One lot of thomsonite specimens, containing cavities of consider- 

 a,ble size and irregular form, contains small stilbite crystals. The 

 inner crust forming the lining of the cavity is made up of a thin 

 layer of minute rhombs of chabazite averaging 0.25 millimeter in 

 size. Upon this rest hemispherical masses of radiating blades of 

 thomsonite 2 millimeters thick. The surface of the thomsonite is 

 covered with small variously oriented, stilbite crystals of the usual 

 form and about 1 millimeter long. These are perfect and show the 

 pyramidal termination with no basal plane. In places, especially in 

 the smaller cavities of the specimens, the thomsonite is lacldng and 

 the stilbite rests directly on the chabazite layer. Under the micro- 

 scope these crystals of stilbite lie on the flat side, which is perpendicu- 

 lar to the optic normal or Y. They are very beautiful objects and 

 show brilliant interference colors. At the position of maximum 

 extinction, especially in convergent light, they show strikingly the 

 complex hour-glass structure illustrated in Figure 4, under stilbite, 

 in Dana's mineralogy. 



