b PROCEEDINGS OF THE NATIONAL MUSEUM. vol.64 



surface. These vary in size and are very irregular in form but 

 apparently they represent casts of the interiors of very irregular 

 bubble cavities probably formed by the sudden expansion of steam 

 in viscous lava. They all preserve the original outside crust of red 

 color which resembles a stain of iron oxide, but which, upon more 

 minute examination, is found to be the thin skin of heulandite which 

 formed the first lining of the cavity. All of these heulandite coated 

 masses are hard and compact, but they vary somewhat in internal 

 structure and composition. Some of them are entirely filled with 

 bluish chalcedony and quartz, others are mixtures of mordenite and 

 chalcedony or of mordenite and quartz, while a majority contain only 

 pure mordenite The mordenite of the latter varies from distinctly 

 radial fibrous to very compact in structure The third analysis was 

 made upon such a nodule which was exceedingly tough and difficult 

 to break. Under a lens this shows a confused and interwoven 

 fibrous structure and when examined under the microscope its 

 structure is finely felted fibrous. The mean index of refraction, the 

 only one determinable, is 1.473. This description applies equally 

 well to a type specimen of How's original mordenite from Nova 

 Scotia in the Roebling collection which has a mean index of refraction 

 of 1.473 and a birefringence of about 0.005. The material from this 

 nodule which was analyzed was shown to be pure and free from 

 extraneous substances by optical examination. The analysis gave 

 the following results : 



Analysis (S) and ratios of compact nodule of mordenite. 



The fourth and last analysis of the Idaho mordenite was made upon 

 several of the small masses from the peculiar amygdaloid rock men- 

 tioned on page 3. This rock consists of granular lava having half 

 the volume occupied by gas cavities filled with pebblelike masses a 

 few millimeters in diameter which differ strikingly in color. Mor- 

 denite occurs alone in this rock as snow-white masses of very compact 

 texture, also as larger and looser textured flesh-pink masses, the 

 color being due to a very thin outer coating of heulandite, and as 



