AHT. -8 



MINERALOGY OF TEIASSIC LIMESTONE SHANNON 



23 



the preceding as to whether 

 the modifying pyramids and 

 dome be made positive or 

 negative. The five forms, 

 however, are fairly common 

 on datolite as positive forms 

 and are much rarer as nega- 

 tive forms, hence the orien- 

 tation adopted was as drawn. 

 The crystal is unique, how- 

 ever, since in datolite nega- 

 tive hemipyramids are usu- 

 ally developed much more 

 frequently and in greater 

 number than are positive 

 hemipyramids. The crystal 

 shown in figure 4 gave the 

 angles of the following table. 

 As in the first table above it 

 was found best to measure 

 this with the a axis vertical 

 and the table is composite, 

 the indices, etc., being those 

 for the Dana orientation, 

 while the angles are for the 

 corresponding forms taken 

 from the Winkeltabellen. 



Fig. 3. — Datolite. Similar to figure 



2 BUT having some NEGATIVE PTEAMIDS 

 NOT REPRESENTED BY CORRESPONDING 

 POSITIVE FORMS 



Measurements of datolite, Figure 4 



