FENNER, THE WATCHUNG BASALT 



151 



' aggregates, however, in which it is mixed with other minerals, afford 

 better subjects for the microscopic study of replacement phenomena. 



In slide 63, in which datolite is in association with albite, as previously 

 described, the normal appearance of the unaltered mineral is shown. It 

 forms an aggregate of interlocking anhedral crystals, approximately equi- 

 dimensional. The character is biaxial and negative, optic angle large, 

 dispersion formula p> v. The manner in which albite is replaced by 

 datolite in slides 63, 70 and 123 was described under albite, and the 

 replacement of quartz in 70 under quartz. The last is shown in fig. 8. 



The relations of datolite and prehnite are shown in several slides. The 

 deposition of the two minerals is considered to have been approximately 

 contemporaneous, on account of similar relations to preceding and follow- 



FiG. 17. Corroded crystals of prehnite (Pr) in the midst of datolite (Da). 



X 35. Slide 67. 



ing species, but in the cases where it has been possible to determine the 

 relative succession of the two, datolite appears to be later than prehnite. 

 In slide 101, small crystals of datolite are perched upon a radiate group 

 of prehnite crystals in contact with stilbite, as shown in fig. 16. From 

 their position, it seems necessary to suppose that they were deposited upon 

 previously-formed prehnite. In 67, radial and columnar prehnite is mixed 

 with datolite in an involved manner. In places, the prehnite shows strong 

 corrosion, and outlying fragments are on the verge of disappearance. 

 Small portions have been isolated by datolite. The apparent relations are 

 shown in fig. 17. In 74, there is undoubted replacement of prehnite by 

 datolite. 



The relations of datolite to the zeolitic minerals seem very plain. It 

 appears that under normal conditions datolite was in process of removal 



