20 



PKOCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 66 



One of the best of the minute crystals yielded approximate meas- 

 urements sufficient to identify the forms, after the other properties 

 had served to identify the mineral. The angles are given in the 

 following table. The flat lath-like form is due to the predominance 

 of the prism /(310). The habit of the measured crystal is shown 



in figure 1. 



Measurements of vein diopside, Figure 1 



ANHYDRITE? MOLDS 



Many of the specimens of datolite from Leesburg contain tabular 

 hollow cavities, now empty or partially filled with a late deposit 

 of calcite, which evidently owe their form to crystals of some 

 mineral which has now been completely removed. Many of these 

 cavities are mere gashes showing the mineral to have been very thin 

 tabular and they have often formed parallel aggregates or slightly 

 divergent sheaves of plates and in a few cases rosettes of thin tables 

 ladiating from a center. In size the gashes range from exceedingly 

 thin ones with a length of 1 or 2 millimeters to an extreme size, in 

 those examined, of about 3 by 20 millimeters in cross section. The 

 cavities are rectangular in cross section and no impresions of termi- 

 nations could be made out (See pi. 3). 



These are entirely similar to the tabular empty cavities so common 

 in zeolite specimens from the New Jersey localities and to similar 

 impressions or molds which have also been noted at Westfield and 

 elsewhere in Massachusetts and at Meriden, Connecticut. At some 

 of these places they are associated with anhydrite which partly 

 fills them, and it seems altogether probable that in all of the local- 

 ities, including that at Leesburg, the cavities are the impressions of. 

 anhydrite crystals. 



' The minerals which preserve the cavities are datolite and calcite 

 of the generation which formed immediately after the datolite but 

 the anhydrite was removed earlier than the deposition of the later 

 globular calcite which occurs in the crystal molds. The main 

 generation of the datolite is later than the anhydrite but the cavities 



