70 



PEOCEEDINGS OF THE NATIONAL MUSEUM. 

 Measurements of datolite, Figure 21. 



VOL. (3ff 



The forms r (230) and 

 (120) are present as small 

 and dull etched faces. No 

 measurement was obtained 

 on V (Tl 1) , which was iden- 

 tified by its position; e 

 (Tl2) is etched so as to give 

 a reddish signal. All of 

 the other faces are plane 

 and brilliant, giving excel- 

 lent reflections. 



In the late summer, 1923' 

 numerous specimens of this 

 postprehnite datolite were 

 found, mostly granular but 

 some showing free crystals. 

 A specimen handed to 

 me by the owner of the 

 quarry in August, 1923, 

 had been laid aside by the 

 quarry foreman, and what 

 part of the quarry it came from was not known. This had a relatively 

 large cavity filled with datolite surrounded by an earlier crust of prehn- 

 ite. The datolite crystals of this specimen are the most flattened 

 crystals of this mineral which have come under my notice. The largest 

 of these may reach a breadth of 15 mm. with a thickness of only 1 mm. 

 They are imperfectly developed and it was found impossible to orient 

 them except by placing them on the flat side and examining them 

 optically. The emergence of an optic axis nearly perpendicular to 

 the flat face identified it as the dome x (102), and this, together with 

 the position of the optic axial plane, served to orient them. It was 



Fig. 22.— Datolite of second generation showing pro- 

 nounced TABULAR development PARALLEL TO X (102). 



