No. 2385. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 495 



mathematical laws. Such striated contact surfaces are well shown 

 in the photograph (pi. 104). In less frequent instances the growth 

 tendency was exerted more strongly in one direction and resulted 

 in sheaves of platy crystals, grown together in more or less parallel 

 position, the plane of contact in many examples being approximately 

 parallel to £ (T02). 



The crystals vary from transparent to almost opaque. The trans- 

 parent crystals when first exposed to light are rather deep yellow 

 green in color, but the color fades gradually on exposure to light 



Figs. 1S-21.— 18, Clinogeaphic detail of crystal 41 showing the new foems fci (791), j (142), f (143), 



J (263), # (296), (295). A (2.10.5), AND J (229). 19, CLINOGEAPHIC DETAIL OF CRYSTAL 10 SHOWING EAEE 

 AND NEW FOEMS g) (6.15.2), h (9.16.2), g x (262), t (232), [ (242), #(296), AND 8 (2.12.10). 20, CLINOGRAPHIC 

 DETAIL OF POETION OF CEYSTAL 44 SHOWING NEW FOEM u (211). 21, CLINOGEAPHIC PEOJECTION OF 

 CRYSTAL 43 SHOWING UNUSUAL FOEM s (530). 



until they become almost colorless. The opaque crystals are pure 

 white in color and possess a peculiar porcellanous appearance. 



Cleavage was not observed, unless some faint rifts, which are 

 parallel to the pinacoid a (100) and give iridescent reflections, indicate 

 a very obscure cleavage parallel to this plane. Whitlock describes 

 his crystals as showing distinct cleavage parallel to a (100) and 

 parallel to c (001) somewhat less perfect. The present observations 

 do not confirm these results. Several large transparent crystals were 

 heated to various temperatures and chilled with water in an attempt 

 to develop cleavage, but the crystals became filled with ramifying 



