152 ANNALS NEW YORK ACADEMY OF SCIENCES 



during the entire period of formation of zeolites. Such a condition does 

 not appear surprising, if the inferences as to the source of the boric acid 

 are correct. The amount deposited by sublimation in the crevices of the 

 lava would be a definite quantity, and no further supply would be avail- 

 able during progressive alteration of the basalt. Under the continual 

 leaching of the uprising waters, it should therefore gradually disappear. 

 In section 95, the relations of datolite and chabazite are well shown. 

 The rock is a quickly chilled basalt, in which phenocrysts of plagioclase 

 and diopside are scattered through a groundmass of feathery microlites. 

 It contains numerous irregular cavities which probably represent steam 

 vents. The chief filling of the spaces is chabazite and heulandite, but 

 along the margin of the amygdules are numerous corroded crystals of 



Fig. 18. Datolite (Dat) replaced by chabazite (Ch) and calcite (Cal). X 35. 



Slide 95. 



datolite. The manner in which the datolite is yielding to chabazite is 

 sketched in fig. 18. Frequently the datolite crystals retain strong sugges- 

 tions of their outward form, but the core is gone and its place is occupied 

 by chabazite. Between the datolite and the less altered basalt, finely 

 granular, muddy-looking calcite has entered in the manner which has 

 been found to be so frequent. 



In 110, there is a similar replacement of datolite by stilbite (fig. 19). 

 It is noticeable in many cases, as the figures indicate, that parts of the 

 datolite crystals are unattacked and retain their proper outlines, while 

 other parts are deeply corroded. 



In 94, there is replacement by heulandite. At one point, there is a sug- 

 gestion in the structure of the datolite, heulandite and associated calcite, 



