ABT.19. MORDENITE— BOSS AND SHANNON. 6 



very fine grained and of green, gray, or white color, predominate. It is nicely 

 bedded, in places thinly laminated. , ^, , .. 



The hurried observations afforded little idea of the thickness of the deposits, 

 although it must unquestionably be hundreds of feet. 



Fossils were not found in this area, but the topographic relations of these 

 beds are similar to those at Salmon, which were assigned to the Miocene on 



floral evidence. -.at 



Julien * has published petrographic descriptions of several volcanic tuffs from 



Challis. 



It seems most probable that the bed containing the mordenite and 

 other minerals here described is an exceedingly vesicular andesite 

 flow interbedded with the Miocene lake sediments. The larger geodal 

 masses of the zeohtes have some inclosing rock attached, but this is 

 friable and sandy textured and falls to pieces except where it has been 

 silicified-by processes attending the deposition of the minerals in the 

 cavities One spechnen which was labeled "country rock" consists 

 of very numerous small pebblehke amygdules fiUed with mordenite, 

 heulandite, and diabantite or a related chlorite, in a friable greenish 

 matrLx. A thin section of this rock, which below is frequently 

 referred to as " the amvgdaloid," shows the matrix to consist of pheno- 

 crysts of augite in a glass base. FuUy half of the volume of the rock 

 is occupied by the filled vesicles, as shown in the two middle illustra- 

 tions of plate 2. The forms of these smaU amygdules vary from 

 spherical to irregular, but show them unquestionably to be gas cavi- 

 ties rather than replacement nodules in a tuff aceous rock. 



MORDENITE. 



Mordenite was originaUy described from Nova Scotia, where it has 

 been found at several locaHties, but only in relatively small amount 

 at each, as compact masses with confused fibrous structure fiUing 

 small cLvities m lavas. No finely fibrous or cottony material is 

 known from Nova Scotia, although, as shown by Schaller in the paper 

 previously cited, the flokite from Iceland, which has such structure, 

 is identical with mordenite. The mineral seems to be present as finer 

 specunens and m greater abundance at the Idaho locahty than at any 

 other known occurrence. In the Idaho spechnens exammed the 

 mordenite occurs in a variety of forms ranging from fine cottony felted 

 aggregates through radial-fibrous masses to dense compact porce- 

 laneous materials. Most abundant and conspicuous are the large 

 geodal masses of fine cottony material which reach a diameter of a 

 foot or more. These vary from round to irregular in cross section 

 and consist usuaUy of a thin shell of chalcedonic material fiUed with 

 the fight tufted fibers of the mordenite. Although this cottony ma- 

 teriafappears entirely homogeneous to the unaided eye, it aU con- 

 tains gritty particles which can be felt when it is crushed, and upon 



• Alexis A. Julien. Volcanic tuffs from ChaUis, Idaho. Trans. N. Y. Acad. Sci., vol. 1, pp.49-53, 1882. 

 20183— 25— Proc.N.M.vol.64 29 



