34 PEOCEEDINGS OF THE NATIONAL MUSEUM VOL.71 



erties, and it is probable that all of the fibrous white material in 

 the hornfels is this mineral. This zeolite is of common occurrence in 

 the best vesuvianite specimens not only in the hornfels but also in 

 the cavities with the vesuvianite where it is always in small amount 

 and never conspicuous. It forms thin fibrous crusts varying from 

 dense and tough to aggregates of tufted cottony fibers. It rests upon 

 vesuvianite and garnet but is clearly older than the chabazite which 

 occurs in the same cavities. It is recognized, microscopically, by its 

 negative elongation, small inclined extinction, medium low birefrin- 

 gence and index of refraction which approximates 1.51. 



In the scapolite specimens the scolecite is even less well charac- 

 terized. The best large specimens of scapolite (No. 84567) show- 

 no scolecite to the naked eye but look like large columnar masses of 

 mizzonite with the interstices filled with calcite. Under the micro- 

 scope, however, the mizzonite is seen to in large part be replaced by 

 fibers parallel to the vertical axis, which have negative elongation, 

 index approximating 1.515 and where coarse enough these show 

 evidence of twinning and inclined extinction. Another large group 

 of scapolite specimens consist of mizzonite prisms the interstices 

 being filled with pale gray green byssolite. These masses of scapolite 

 are also very largely replaced by the fibrous scolecite. These speci- 

 mens (No. 84566) also show nearly lusterless masses of radiating fine 

 white fibrous scolecite up to 2 cm. in maximum diameter. This 

 fine-fibrous material is biaxial negative with small inclined extinc- 

 tion, a=-- 1.510, ^ = 1.515, approximately. Occasionally the zeolite 

 coarsens to lustrous acicular prisms. Under the microscope these 

 all show twinning lamellae like a plagioclase and all lie on a cleavage 

 face approximately parallel to the optic axial pane. They show 

 inclined extinction, Z A <?— 14° with a= 1.512, y— 1.518, approxi- 

 mately. 



The scolecite occupying cavities with other zeolites is similar to 

 the foregoing. In one specimen (No. 84545) a little white-radiat- 

 ing compact-pearly material resting on vesuvianite is clearly older 

 than both stilbite and chabazite. A specimen (No. 84578) consist- 

 ing largely of minute rosettes of pearly pale brown blades of stilbite 

 has the latest cavities filled with radiating compact fibrous scolecite, 

 apparently later than the stilbite. In another specimen sheaves of 

 stilbite up to 7 or 8 mm. long (No. 84581) are underlain by rosettes 

 up to 1 cm. across of fairly coarse radiated pearly needles. Under 

 the microscope these fairly coarse needles show the twinning and 

 inclined extinction, X A (?=12°. The mineral is biaxial negative, 2V 

 moderate, refractive indices a= 1.510, ;8= 1.515, A= 1.516. Another 

 single specimen shows large crystals of sahlite projecting into a 

 mass of stilbite and fine fibrous white scolecite. The stilbite forms 

 minute pale brownish bladed rosettes apparently older than the 



