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ANNALS NEW YORK ACADEMY OF SCIENCES 



of radiating crystals, the individuals increasing in size outward. There 

 was also probably a small amount of the unknown mineral whose removal 

 by solution left the prismatic cavities previously described. In this case, 

 the cavities have been refilled by heulandite, but are still sharply out- 

 lined. In other parts of the slide, quartz is giving way to heulandite, but 

 in such cases the habit of the heulandite is different from that shown in 

 its deposition in the open cavities. 



In fig. 11, some indication of the radiating form of the quartz can be 

 distinguished, and outside the field the center of radiation is plainly 

 seen. The replacement by cha])azite has taken a most irregular form, but 

 the influence of the crystallographic character of the quartz upon its 

 solution and removal are perceptible in places. 



Fig. 12. Replacement of quartz (clear) by heulandite (stippled). Arrows show 

 direction of vertical axes of quartz crystals. Many of the crystallographic 

 lines approach parallelism, but few are strictly pai-allel. X 35. Slide 77. 



In fig. 12, the major portion of the field is occupied by a single quartz 

 crystal, in nearly basal section, as is shown by the interference figure. 

 Surrounding it are other quartz crystals of various optical orientation. 

 Eeplacement by heulandite has taken the form shown, extremely irregular 

 but yet plainly governed more or less by differences of solubility in the 

 quartz. 



Plate XI, fig. 4, illustrates one of the complex zeolitic groupings often 

 found. Even at this stage of alteration, vestiges of the original texture 

 of the rock and of the results of mechanical action upon it can be seen 

 in many places. Phenocrysts of diopside are quite numerous outside of 

 the field illnstratod. Traces of plagioclase laths can be made out, and the 

 subcircular chlorite nodules due originally to resorption of olivine are 



