1G8 ANNALS NEW YORK ACADEMY OF SCIENCES 



frequently in cockscomb growths, predominate. Stilbite is prevailingly 

 of a light straw color, and the usual sheavelike clustering is the common 

 mode of growth. 



In the thin sections, heulandite and stilbite bear such a close resem- 

 blance to each other that great care must be used to distinguish them. 

 There are two tests, however, depending upon optical characteristics, 

 which are of such a nature that one is especially applicable to those sec- 

 tions in which the other fails to give good results. The first is that 

 heulandite is positive in character and stilbite is negative, but in sections 

 in which the lamellar cleavage is most apparent, this test fails. In such 

 sections, however, the direction of the plane of the optic axes can usually 

 be ascertained. In heulandite, this plane is perpendicular to the cleavage, 

 but in stilbite, it is parallel. 



Fig. 25. Replacement of ehabazite (Ch) by heulandite (H). X 32. Slide 95. 



There is some yariability in the size of the optic angle in both heulan- 

 dite and stilbite. It is never large but may become very small. In 121, 

 a section of stilbite gives practically a uniaxial cross. The positive or 

 negative character, however, does not seem to vary. In ehabazite, the 

 figure is usually biaxial and always positive so far as tested. In some 

 crystals, the interference figure is distinct and the optic angle rather 

 large, but, in many, the birefringence is so weak that it is difficult to 

 perceive any figure whatever. 



The relations of ehabazite to heulandite are shown m a number of 

 slides (for example, 95, 108, 94, 51a, 77 and 128), and it appears that 

 ehabazite always precedes heulandite. With growth and advance of the 

 later mineral, the ehabazite breaks down along the border in a character- 

 istic manner. An example is shown in fig. 25 (slide 95). In the upper 



