N0.23S5. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 483 



average diameter of the crystals measured was approximately 1 cm., 

 but crystals up to 3 cm. in diameter were measured at times. In 

 these cases it is often necessary to shift the crystal backward and 

 forward during measurement, thus slightly affecting the accuracy of 

 the measurements, but the close agreement between the angles meas- 

 ured and the calculated angles indicates that in most cases the errors 

 due to this cause are small. Many of the rarer forms which are 

 present as small faces are characteristically etched and dull, and 



Figs. 5-6.-5, orthographic and clinographic projections of symmetrically developed datolite 

 crystal of the acute habit designated type 2 by kraus and cook. a commonly occurring habit 



AT WESTFTELD. 6, ORTHOGRAPHIC AND CLINOGRAPHIC PROJECTIONS OF A SYMMETRICALLY DEVELOPED 

 CRYSTAL OF THE PRISMATIC TYPE -WITH LARGE DEVELOPMENT OF <J (100), DESIGNATED TYPE 3 BY KRAUS 

 AND COOK. SHOWS UNUSUAL FORM <p (101). 



many faces which are clearly visible reflect no light at all. Many 

 measurements are rendered inaccurate, even though the signal was 

 clearly discernible, by the fact that the illumination was insufficient 

 to render the cross hairs visible and the signal could only be brought 

 to an approximation of the center of the field. In many cases small 

 and partly etched faces which give only a faint signal are hard to 

 accurately measure, because they fall so near other larger and highly 



