FENNER, THE WATCH UN G BASALT 



169 



left-hand portion of the sketch, the chabazite retains its normal outline, 

 but at most places along the border it has a brecciated appearance. This 

 feature is observable in a great number of instances. 



The replacement of chabazite by stilbite usually takes a similar form. 

 It is well shown in slides 110, 108, 80 and 121. In slide 108 (fig. 15), 

 the chabazite is cut by distinct veins of stilbite, portions of which are 

 shown in the sketch. The irregular boundaries of the veins are probably 

 determined both by differential solution of chabazite and by the crystal- 

 lizing tendencies of stilbite. In fig. 26 (slide 121), also, stilbite is ad- 

 vancing into chabazite in irregular veinlike form. 



In slide 110, isolated crystals of chabazite surrounded by stilbite show 

 deep corrosion. At one point in the slide, the phantom forms shown in 



Fig. 26. Replacement of cbabazite (Ch) by stilbite (St) and of both by 

 natrolite. The natrolite needles have shot forth with apparently little oppo- 

 sition across areas of chabazite and stilbite. and they have also worked in 

 along cleavage lines of chabazite. X 35. Slide 121. 



fig. 27 appear. These are simply inclusions in stilbite, but the forms out- 

 lined are plainly those of preexistent crystals, which are believed to have 

 been chabazite. 



Heulandite and stilbite frequently occur together, but it is rather rare 

 for either to show unmistakable corrosion at the contact. In slide 108, 

 however, the evidence is fairly good that heulandite inclosed within stil- 

 bite is being replaced by the latter. In 110 and 92 also, some confirma- 

 tory evidence is found. The more usual form of contact is shown in 8S, 

 where it is perfectly sharp. In several hand specimens, more indubitable 

 evidence is given. The heulandite appears fresh and glassy but is crusted 

 with stilbite. The inference from the phase rule is that heulandite and 



