NO. 2345 CONTRIBUTIONS TO MINERALOGY— SHANNON. 447 



The crystals reach an extreme diameter of about 1 centimeter. 

 They are colorless and rather lacking in brilliancy, so that they look 

 like the calcite of the other parts of the quarry in color and luster 

 and none of the specimens were identified as datolite until studied 

 in the laboratory. The crystals on the five specimens collected are 

 all remarkably alike in form. The habit is rather thick tabular 

 parallel to the front pinacoid, the appearance being essentially as 

 shown in orthographic and clinographic projection in the drawings 

 figure 13 a and 5. These crystals resemble those from Westfield, 

 Massachusetts, except in this large development of the front pinacoid, 

 but such tabular crystals have not been observed at Westfield. 



The forms observed on the crystals are : 



Pinacoids: Pyramids: 



a (100). n (111). 



c (001). € (112). 



Domes: X (113). 



t (013). K (115). 



g (012). a (124). 



m^ (Oil). (121). 

 Prisms: 

 m (110). 



o (120). 



a (100) is invariably the dominant form, being present as a broad 

 and relatively smooth face, characteristically pearly in luster, as from 

 reflection by parting or cleavage rifts parallel to this plane. 



c{001) is prominent as a bright but somewhat irregular face, 

 characteristically striated parallel to its intersection with the pyra- 

 mid x (T 15). 



2^(013), <7(012) and ^^^(Oll) are prominent faces, especially the 

 two latter, and yield clear brilliant signals ; 7.(115) , X (113) , and e (112) 

 are present usually as bright small faces yielding good reflections. 



m(llO) is represented by small brilliant planes. 



n{lll) occurs only as a narrow but bright line beveling the edge 

 a(lOO) Am^(Oll). 



^(121) forms a narrow line beveling the angle m(llO) Amx(Oll). 



<?(120) forms a small triangular face, which with P(121) is al- 

 ways etched too deeply to reflect any light at all. These dull forms 

 were identified by their zonal relations. 



a (T24) occurs occasionally as a small brilliant plane beveling the 

 edges (Tl2)A ^(012). 



