FENNER, THE WATCHUNG BASALT 133 



"of 10-15 mm., and much larger ones occasionally appear. It is generally 

 colorless, but not infrequently amethystine, and more rarely smoky. The 

 faces shown are usually simple combinations of prism and pyramid. In 

 addition to the groups of euhedral crystals which are most common, 

 granular aggregates sometimes occur, as well as deposits of chalcedonic 

 silica. 



Frequently specimens are found which show numerous cavities, indi- 

 cating plainly the removal of groups of crystals of some mineral which 

 was intergrown with the quartz. Most of the cavities are diamond-shaped 

 in cross-section, but others are apparently rectangular. They are un- 

 doubtedly due to the removal of groups of crystals, but the forms do not 

 suggest those of any of the species known from these localities. It ap- 

 pears most probable that among the various transformations experienced 

 one or more minerals disappeared completely. There are indications that 

 formation and removal occurred early in the history of the series, for at 

 times the cavities have been filled with later-deposited quartz. 



Replacement of Quartz 



In slide 70, of which the hand specimen consists principally of dato- 

 lite, heulandite and natrolite, the datolite, under the microscope, is seen 

 to contain at one place a small group of albite crystals, greatly corroded 



Fig. 8. Fragments of quartz outlining former crystal, now almost completely 

 replaced by datolite. X 35. Slide 70. 



as previously described, and scattered through the datolite are minute 

 remnants of quartz. One occurrence takes the form shown in fig. 8. It 

 is clear that the scattered particles are portions of the pyramidal termina- 



