ABT.19. MORDENITE — BOSS AND SHANNON. 9 



servation despite the fact that Thugutt had described a "ptilolite" 

 from Seiser Alp which showed inclined extinction. With this in 

 mind the Idaho material was very carefully examined, its composi- 

 tion and refractive indices being identical with those given for fiokite. 

 Since the fibers are very thin and the birefringence is very low an 

 extinction obliquity of several degrees might readily evade observa- 

 tion. Such a small angle might also be attributed to lack of adjust- 

 ment between the cross hau-s and nicols of the microscope. After 

 specially adjusting the microscope the Idaho material was found to 

 give, as the mean of about 20 good measurements, an extinction 

 inclination of 3^°, measured from the elongation of the fibers. In- 

 stead of containing errors of observation, Miss Callisen's work on 

 flokite appears to be a very commendable and highly exact research 

 which has furnished the key for the interpretation of the group. 



The Idaho mordenites are interesting especially as furnishing a 

 complete gradation between the physically very dissimilar compact 

 mordenite from Nova Scotia and the cottony fibrous ''fiokite" from 



Iceland. 



HEULANDITE. 



Heulandite occurs in the specimens as a complete filling of small 

 vesicles in the rock, as drusy linings of cavities, and as thin crusts 

 forming the first lining of cavities later filled with mordenite, 

 chalcedony, or quartz. The characteristic flesh-red masses in the 

 amygdaloid consist of fine transparent heulandite, and many of these 

 small masses are hollow and present fine terminations of acute wedge- 

 shaped crystals, the habit being as shown in figures 1 and 2. These 

 show very well developed cleavage parallel to the pinacoid b(OlO) as 

 well as some other less perfect cleavages at right angles to this plane. 

 Usually two or several crystals are grown together in more or less 

 parallel position, but the tendency to parallel growth is not so con- 

 spicuous as usual in the heulandite of this type. A mp,jority of these 

 pebbles are not hollow, but are solid masses of the heuandite. Those 

 which do show open centers often have a few fine fibers of mordenite 

 extending across the central cavity. Other heulandite-lined vesicles 

 in the same rock are completely filled with closely packed cottony 

 mordenite and the faintly pink to salmon colored masses which fill 

 many of the larger bubble cavities are compact mordenite with a 

 thin outer skin of heulandite. The numerous irregular masses which 

 have weathered free from the inclosing rock, and which consist of 

 compact mordenite, of mordenite saturated with quartz or with 

 chalcedony, or of quartz or chalcedony alone, all have a brick-red 

 outer coating which resembles a stain of ferric oxide, but which, 

 when examined with a lens, is found to consist of a thin outer crust 

 of crystalline heulandite. All of the larger geodal masses of mor- 

 denite, of whatever type, have a thin deposit of heulandite next the 



