494 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



vol. 59. 



are bounded by crystal planes. In many ways the thicker crusts 

 of datolite resemble the quartz combs of veins. They differ, how- 

 ever, in that each crystal repelled those adjoining it and preserved 

 its individuality while growing upward, hemmed in on all sides, 

 and instead of interlocking firmly in the crusts, the masses of crystals 

 can often be separated with the fingers. Normally mutual interference 



Figs. 16-17.— 16, Orthographic and clinografhic projections of crystal 10 showing form and 

 development, also the forms i (167) i> (205) and the vicinal form (16.0.39). 17, orthographic 

 and clinographic projections of crystal 51 showing the new forms x (343), u (344), 93 (345) and 



O (134). 



seems to have resulted in pressure exerted nearly equally in all direc- 

 tions, so that many crystals are now more or less columnar, with the 

 termination only bounded by crystal planes. The surfaces of contact 

 with adjoining crystals show peculiar striations, doubtless resulting 

 from oscillation exerted by growth pressure and following definite 



