172 ANNALS NEW YORK ACADEMY OF SCIENCES 



chabazite crystals, and the manner in which it replaces chabazite has been 

 described. 



In slide 120, it appears to be replacing heulandite, and in a great many- 

 hand specimens, it is seen to have been deposited upon previously formed 

 heulandite. In one or two instances, it has been found resting upon 

 stilbite. 



In 82, it is found in contact with natrolite, and it is quite certain that 

 natrolite crystals are advancing through the laumontite. On the other 

 hand, in a large specimen, the appearance is decidedly in favor of the 

 interpretation that laumontite is later than natrolite. It is probable that 

 the relations of laumontite and natrolite are similar to what was found for 

 heulandite and analcite, that is, the periods of the two overlap, and rela- 

 tions in a given case depend upon slight differences in concentration of 

 lime and soda. 



Similarly, in 128, it appears that calcite replaces laumontite, but in a 

 great number of hand specimens, laumontite groups are perched upon 

 calcite crystals. From this last occurrence it appears that laumontite 

 forms one of the end members of the zeolite series. The period did not 

 come to a sudden end, but a transition period intervened, during which 

 both laumontite and calcite were deposited. 



Scolecite 



In a number of slides (89, 736, 68, 71, 143), renmants of some mineral 

 are found which is believed to be scolecite. It gives inclined extinction 

 and is probably triclinic. Birefringence about 0.007, refraction > 

 natrolite and < balsam, elongation negative, optical character indeter- 

 minate. 



It has been found only in association with natrolite, which replaces it 

 in the manner shown in fig. 29. Its exact position in the series is there- 

 fore uncertain. 



Natrolite 



Natrolite is one of the most abundant zeolites and is found in a groat 

 many of the thin sections. There is little to add to what has already 

 been written, for the reason that, like laumontite, it is one of the end 

 members and replaces practically 'everything else, and these replacements 

 have been described under the corresponding minerals. In its relations 

 with calcite it appears to differ from laumontite, for wherever the two 

 come together, the natrolite always appears earlier. The replacement is 

 well illustrated in 121. Xatrolite needles which have advanced through 

 stilbite and chabazite are wiped out abruptly by encroaching calcite. 



