348 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



and analyzed with the results given in column 1 of the table. The 

 material of this analyzed sample was biaxial and negative with the 

 orientation XAc = 3°40', Y = a, Z = 6. The indices of refraction 

 measured are a = 1.470, /3 = 1.475, 7 = 1.475, birefringence, 0.005. 



The second sample analyzed came from a pure nodule about 8 cm. 

 in maximum diameter which has evidently been weathered upon the 

 surface for some time as it is weather stained, the original shell is 

 bleached, and the mordenite is in places stained green by algae. The 

 nodule is red on its original surface from a thin coating of heulandite 

 succeeded inward by a thin shell of chalcedony inside which the mass 

 consists of pure mordenite in fibers extending from all sides toward the 

 center. The fibers are straight and close-packed and reach a maxi- 

 mum length of 2 cm. The color is creamy white, and the luster, on 

 fresh fractures, is faintly silky. The analysis gave the results of 

 column 2 of the table. The refractive indices measured on this ana- 

 lyzed sample are: a = 1.472, 0=1.475, 7=1.476, y- a = 0.004. All 

 of the fibers which are thick enough to give appreciable birefringence 

 show a small inclined extinction. 



Many of the mordenite specimens received are amygdules free from 

 matrix, which apparently were gathered, like so many pebbles, from 

 the 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 very viscous lava. They all preserve the original 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 in composition. Some of them are entirely filled with bluish 

 chalcedony and quartz, others are mixtures of mordenite and chalce- 

 dony or 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 

 How's original mordenite from Nova Scotia. The analysis is given 

 in column 3 of the table. 



The fourth analysis was made upon white pebble-like masses from 

 an amygdaloid rock which is very vesicular, having half the volume 

 occupied by small rounded cavity fillings of various colors (pi. 1 1, 

 center). These include dull-green masses of scaly chlorite, flesh-red 

 masses of heulandite, pink masses of mordenite coated thinly with 





