No. 2385. CRYSTALLOGRAPHW STUDY OF DATOLITE— SHANNON. 503 



c (001), the basal pinacoid, is seldom free from irregularities. 

 These most frequently take the form of narrow depressed "ruled" 

 lines in three directions, which are parallel to the unit prism m (110) 

 and the orthopinacoid a (100). These sometimes are not etched, but 

 are merely shallow depressions. They form, however, lines suscep- 

 tible to attack by etching agents and are often greatly deepened by 

 natural solution. The effect is to divide the face into triangular 

 areas or to produce deep triangular pits or depressions. This plane 

 has been one of the most easily attacked by corroding solutions. 

 Sometimes this pinacoid is wholly dull. The characteristic three 

 directional lines at times look like traces of cleavage, but no cleavage 

 parallel to these directions could be proven to exist in the crystals. 

 Such triangular rulings were artificially produced by the action of 

 dilute hydrochloric acid on a crystal which did not originally show 

 them. 



x (102), which in the majority of the crystals is one of the most 

 prominent faces, is most frequently dull, as in datolite from other 

 localities. At times this dullness appears as a uniform etching over 

 the whole face, giving the appearance of finely ground glass. More 

 frequently the etching has taken the form of broad irregular lines, 

 which divide the face into lozenge-shaped patches, separated by 

 broad depressed channels. In other instances there appears a large 

 irregular etched depression in the center of the face, from which 

 crooked lines ramify in all directions. When the faces of this form 

 are unetched they are horizontally striated by oscillatory lines, which 

 increase in number and distinction toward the top of the face. 



e (Tl2) and also X (113) and to a less extent fx (T14) are character- 

 istically marked by an oscillatory horizontal striation. This is a 

 most constant and diagnostic peculiarity of the important form 

 e (Tl2), which is always lustrous and free from corrosion. 



o (120) and the pyramids of the same vertical zone as U (123), 

 Q (122), (121), and M (T22) are almost invariably dull by the 

 presence of "ground glass" etched surfaces, which, while variable in 

 degree, are almost never wholly absent. This etching renders these 

 faces readily identifiable, thus greatly facilitating proper orientation. 



v (Til) and b (223) are often deeply etched to a uniform dull sur- 

 face, which often reflects little if any light. Some of the etched faces 

 have a peculiarly iridescent luster. The etching of these forms is 

 very variable in degree and is frequently not present. 



m (110) and n (111) are often ruled by parallel grooves due to oscil- 

 lation between the two forms. These grooves are sharp reentrant 

 angles and are not accompanied by any rounding, so that both faces 

 give perfectly sharp signals. While not invariably present, these 

 grooves serve immediately to identify these faces when they do occur- 



