No. 2385. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 493 



GENERAL FEATURES. 



The datolite of the Westfield specimens occurs commonly as thick 

 crusts of comparatively large crystals, lining open spaces in the 

 basaltic rock, although rather dense veins and masses of granular 

 datolite occur occasionally. There appears to be but one generation 

 of the mineral and no other mineral is contemporaneous with it. 

 The datolite in all parts of the area appears to have been deposited 

 at the same period and under very similar conditions as regards 

 temperature, pressure, and composition of solutions. The crystals 

 vary from 1 millimeter to approximately 10 centimeters in diameter, 

 the average diameter being around 1 centimeter. The crystals of 



Figs. 13-15.— 13, orthographic and clinographic projections of crystal bo, showing unsymmet- 

 sical development, etching of c(001), and striation of 8 (205); also the rare and new forms 



. S (302), I (203), u (116), N (126), £(138), AND ji (3.6.30). 14, CLINOGRAPHIC PROJECTION OF CRYSTAL fill, 

 SHOWING PARALLEL GROWTH WITH REENTRANT ANGLE RESEMBLING TWINNING. ALSO SHOWS CHARAC- 

 TERISTIC "RULING" OF THE BASAL PINACOID. 15, CLINOGRAPHIC PROJECTION OF CRYSTAL 39 SHOWING 

 PROMINENT DEVELOPMENT OF S (302) AND £ (102). 



any given cavity or specimen, which are usually of approximately 

 the same order of magnitude, appear to have grown simultaneously, 

 very closely crowded together, and at about the same rate, and only 

 rarely has an individual outstripped its neighbors and assumed 

 a large size at their expense. The close crowding of the crystals 

 has been almost universal, and they are consequently not nearly so 

 well developed as would have been possible had they been more 

 sparsely distributed over the base to which they are attached. 

 Only the free surfaces projecting into the open portion of the cavity 



