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ANNALS NEW YORK ACADEMY OF SCIENCES 



lost its normal isotropic character. The natrolite, on the contrary, looks 

 perfectly fresh. The outline of the contact is very irregular, but it is 

 observed that at some distance back in the natrolite, the crystals of the 

 latter suddenly change from a finely felted to a coarsely crystalline char- 

 acter. The border between the two varieties is easily seen in the thin 

 section with the naked eye and suggests the outline of an analcite crystal. 

 It does not seem impossible that the natrolite replaced various minerals 

 (of which laumontite and analcite were two), and that the coarseness of 

 crystallization depended upon the facility with which replacement was 

 accomplished. In portions of the slide where it is plain that laumontite 

 is being replaced, the crystals of natrolite are of a fine character, but 

 where there are reasons for supposing that analcite has been replaced. 



Fig. 24. Porous, decomposed analcite (An), penetrated by needles of natrolite 



(Na). X 35. Slide 82. 



the natrolite crystals reach dimensions many times those shown in the 

 first variety. 



Slide 124 is from the same hand specimen as 82 and shows essentially 

 the same relations. In 73h, there is again a sudden increase in the 

 coarseness of crystallization of natrolite along definite boundaries. 

 Within the masses of larger crystals, there are two extremely irregular 

 patches of a nearly isotropic, spongy mineral which appears to be analcite. 

 This is confirmed in another portion of the slide, where similar forms 

 still show crystal outlines. Eeplacement of analcite by calcite is also 

 shown. In 68 also, a small group of isotropic crystals in natrolite show 

 in places typical outlines of analcite. Portions are corroded, and replace- 

 ment by natrolite is evident. 



