474 A. C. Hawkins — Datolite from North Plain field. 



Fig. 1. 



crystalline mass of the same material, which in turn rests 

 directly against the wall of a 6-inch cavity in the vesicular 

 basalt. Steep white calcite scalenohedra are scattered over the 



datolite crystals. The habit of these 

 datolites is closely similar to that of 

 the crystals from Bergen Hill (Dana, 

 System of Mineralogy, Ed. 6, fig. 4, 

 p. 503), being prismatic, with w T ell- 

 developed basal pinacoid. Speci- 

 men No. 2 shows a few larger crys- 

 tals, averaging in size about 4X&X 

 7 mm . These latter crystals are more 

 strongly pyramidal in development 

 (op. cit., fig. 1). They are slightly 

 deeper in color than those of specimen No. 1, and are not so 

 lustrous. 



The crystals were measured on the two-circle goniometer. 

 Twelve crystal forms in all were observed ; of these, ten are 

 common, being observed on practically every one of the seven- 

 teen crystals examined ; two forms are rare. Unit prism and 

 basal pinacoid predominate, other faces being modifications of 

 this fundamental combination. Some forms, as in the case of 

 the unit pyramid [111], are large but uniformly dull ; others 

 are narrow, some excessively so. Especially is this true of the 

 new clinodome [043]. This form, though" narrow and often 

 rounded or " disturbed," occurs upon the majority of the 

 crystals examined, being thus typical of the locality. 



Though the minute size of faces of [043], which appears for 

 the most part as a narrow, bright line, render its signals often 

 uncertain in position, and though the readings obtained (Ro) 

 vary from 37° 00 / to 38° 43', numerous readings were obtained 

 which are close to the calculated angle for this form, as is 

 also their average, 37° 50'. The form may, therefore, be con- 

 sidered as well established, inasmuch as there are no other 

 forms in the immediate vicinity. 



The normal relations for the clinodomes of datolite now 

 become : 



Letter 



Zone [010 : 001]. 

 e M N* r o 



1 2 



O - - 



2 3 



2 3 inf. 



* New form starred. 



This compares favorably with the normal series N 3 of Gold- 

 schmidt. 



Brown University, Providence, E. I. 



