516 PROCEEDINGS OF THE NATIONAL MUSEUM. voi, 5-.>. 



NEW FORMS. 



Forty-six forms are listed below as new to the species datolite. 

 Some of these, it is true, are open to criticism, inasmuch as the 

 indices are high and seem somewhat irrational, while the greater 

 number of the forms were observed but once or twice each. Two 

 are certainly vicinal and are listed only because they occur as promi- 

 nent faces repeatedly. Other vicinal forms occur frequently in 

 certain zones, but those which were present as narrow faces less than 

 a degree from established forms were ignored. As has been empha- 

 sized above, no form is here listed as new unless there was present 

 on the crystal a distinct and unmistakable face which could not be 

 referred to any established form for this species. Especial precau- 

 tions were taken to exclude from consideration "wild" signals, which 

 were frequently found to be reflected from contact surfaces where 

 adjacent crystals had prevented growth, and also to avoid mistakes 

 regarding faces of small attached crystals, which frequently compli- 

 cate the study of the crystals. Wherever a new form is listed one 

 such doubtless occurs, although it is entirely possible that wrong 

 indices have been derived for some forms, especially those with high 

 indices. The discrepancy which in some cases is apparent between 

 the measured and the calculated angles for new forms may, in some 

 cases, be due to wrong indices having been derived for the form, but 

 in the majority of cases it is due to the difficulty of measuring large 

 crystals requiring much shifting during measurement, and to the 

 failure of the signal to yield sufficient light to illuminate the cross 

 hairs. The dim signals were due to the natural etching, which is so 

 common on the rarer forms. The new forms are described below: 



(E (380) was observed only once as a minute face between o (120) 

 and I (130) on crystal B8, as shown in figure 12. Although small 

 the face was unetched and gave a sharp and brilliant signal. The 

 angles are: 



(380) Measured <? = 30° 08' p = 89° 27' 



Calculated <p = 30° 35' p = 90° 00' 



A= 0° 27' A= 0° 33' 



3 (043). A narrow line face in the side dome zone of crystal 55 

 gave angles nearer (043) than any other simple form. The face was 

 clearly discernible, but was etched so as to give only a very faint 

 signal, which could not be accurately centered. The angles are: 



(043) Measured <p = 0° 00' p = 62°00' 



Calculated <p= 0° 05' p = 59° 21' 



A= 0° 05' A= 2° 39' 



