358 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



amber-brown color averaging 2 mm. on an edge. Under the micro- 

 scope the powdered chabazite shows very low birefringence. Differ- 

 ent grains from the same crystal give both positive and negative 

 biaxial interference figures, with 2V medium large to small. The 

 mean refractive index varies from 1.488 to 1.490. 



BOISE COUNTY 



Specimens labeled as from Warm Springs Creek east of Boise, con- 

 sist of very vesicular basaltic rock, containing abundant cavities up 

 to 2 cm. in diameter largely filled with a white zeolite which is referred 

 to chabazite. 



The rock is a basalt, and contains conspicuous grains of iddingsite, 

 the properties of which are given under that heading. The locality 

 is given as on Warm Springs Creek, which is 

 a tributary to Payette River from the north 

 near the center of the Bear Valley topo- 

 graphic map area. 



The zeolite forms crusts of intergrown color- 

 less to whitish transparent to translucent crys- 

 tals nearly filling many of the cavities. The 

 crystals are not well developed and are grouped 

 in complex twinned aggregates. When the 



Fig. 119.— Chabazite. Simple „ ., ,, , „ , - , 



unit rhombohedron. forms are at all well denned they have a 

 seven devils district, rhombohedral appearance, but no good meas- 



Adams County ... , . . G ,. ,. 



urements could be obtained. Some white radi- 

 ated thomsonite is associated with the chabazite, and in many of the 

 cavities there is a clayey greenish material between the chabazite 

 and the wall of the vesicule. 



Optical determinations gave varied results on material which, 

 under a binocular microscope, appeared to all be the same. Accord- 

 ingly a quantity of the material was crushed and screened and 

 treated with heavy liquids by which process a light, an intermediate, 

 and a heavy product were obtained. 



The light product appeared homogeneous under the microscope 

 and was transparent and colorless with uniformly low birefringence. 

 Dim and rather unsatisfactory figures seem to show a relatively 

 large axial angle with positive optical character, although about one 

 grain out of each six or seven shows an apparently optically negative 

 character. The refractive indices measured were somewhat variable 

 but the maximum value for y was 1.494 and the minimum for a was 

 1.483. Perhaps the average is a = 1.485, 7=1.492. Much of the 

 material shows a lamellar structure resembling the twinning of some 

 feldspars, but very confused and irregular. Many of the grains 

 show rectangular or rhombic boundaries as though determined by 

 cleavages and the persistent difficulty experienced in obtaining 



