170 ANNALS NEW YORK ACADEMY OF SCIENCES 



stilbite cannot coexist in equilibrium except at one fixed temperature, 

 which represents a transition point. Although heulandite might very 

 well persist in an unaltered form beyond this point, it should never be 

 deposited from solution simultaneously with stilbite. 



The replacement of chabazite, heulandite and stilbite by natrolite 

 seems to have been effected with great ease, natrolite needles shooting out 

 across areas of the three 'lime zeolites as if no obstacle were offered. The 

 manner of growth as regards chabazite and stilbite appears in 121 and is 

 illustrated in figs. 26 and 28. In the hand specimen, a polished face was 

 prepared, and it could be seen that the chabazite and stilbite were filled 

 with slender filaments of natrolite radiating from a large area at one side. 



Fig. 27. Phantom crystals of some replaced mineral (probably chabazite) 

 found as inclusions in stilbite. which extends uuiuterruptedly across the 

 areas. X 35. Slide 110. 



In the microscopic section, the effects are similar. The slender needles of 

 natrolite cross areas of the older minerals and also work in along joints 

 of chabazite. 



A similar replacement of stilbite by natrolite appears in 71 and of 

 heulandite by natrolite in 70. These two are from the same rock. In 

 both cases, crystals of the earlier minerals have become isolated in masses 

 of natrolite. The characteristic outlines have been lost, and the borders 

 are so impregnated with natrolite fibres that one mineral seems to fade 

 into the other. 



In 84, the hand specimen consists largely of dense white natrolite, in 

 which small pinkish areas have a geometrical appearance and suggest that 

 some mineral has been replaced. In the thin section, several of these 

 areas appear. It is seen that the pink color is due to separated grains of 



