24 



PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL (50 



In the several variations of the crystals above described the front 

 pinacoid a (100) is marked by vertical striations, which make this 

 form easy of identification and facilitate orientation of the crystals. 



The only other noteworthy feature 

 is the presence of A (210), ordi- 

 narily a rare form on datolite, 

 which is present as narrow but dis- 

 tinct faces on nearly every one of 

 tlie larger crystals. The negative 

 pyramid x^ (342) reported as a 

 new form on datolite from West- 

 field, Massachusetts, is present as 

 a small face on one of the meas- 

 ured crystals from Leesburg. 



Although all of the datolite 

 crystals of a large number of speci- 

 mens from several veins had the 

 general habit above described, one 

 specimen, found loose on the east 

 side of the quarry, contained crys- 

 tals of distinctly different type. 

 This specimen consists of brec- 

 ciated limestone cemented by mas- 

 sive datolite containing vugs lined 

 with colorless transparent crystals 

 up to 6 millimeters in diameter, 

 all of which have the development 

 shown in figure 5. Thfise are, in 

 general aspect, like some crystals 

 found in veins in diabase at Goose 

 Creek quarry. They exhibit sev- 

 eral forms which, while not new, 

 have not been encountered on any 

 datolite crystals which I have 

 heretofore examined. The crystals 

 t're very thick tabular parallel to 

 ;c (102)', while the base c (001) is 

 prominent, and the front pinacoid a (100) is a small and incon- 

 spicuous face. The one of these which was measured gave the forms 

 and angles of the following table, oriented, as drawn, in the Dana 

 position. 



Fig. 4. — Datolite. Similar habit to 

 figure 3 but showing only positive 

 hemipybamios 



