272 B. K. Emerson — The Deerfield Dyke and its Minerals. 



and the two lines run to meet the angles formed by the meeting 

 of the two conical faces, with the central zone. Each of the 

 spindles is tin is made up of three crystals, one on either side of, 

 and one within, the angle of the Y-shaped suture. 



The shaded portion of fig- 

 ure 1 represents such a ver- 

 tical section, in which Do" 

 and Do (=i-i) are the sec- 

 tions of the two conical faces 

 and o" o' {=i-r} that of the 

 central cylindrical face. 



The relations of the three 

 crystals are made clear by 

 fig. 1. Three superimposed 

 prisms (I) having the base 

 {0) in common are revolved 

 on the common vertical axis 

 c to right and left until a 

 prism face of each becomes 

 parallel to the long hori- 

 zontal axis of the third. 

 Thus the two lateral crys- 

 tals form an arrow-headed 



twin with a face of / in commc 

 the middle one so that the sami 

 with it of the latter. If now, 



lateral ["onus be truncated by /-/, 

 by i\ down to their point of conl 

 with a heavier line in the figure, 



and 



a -an 



! face (/) in each is parallel 

 the sharp angle of the two 

 and that of the central one 



o' and o, a plane, marked 

 Its, by whose revolution 

 the short axis a-a, the spindle-shaped twin would be 

 formed. The central crystal has the form of a triangular 

 belt fitted into a similar groove upon the circumference of a 



crystals having 



only the faces /and t'4, would leave for the third crystal only 

 an extremely shallow reentrant angle of 160°, whereas the 

 latter penetrates between the two in an angle of about 41°. This 

 is because the obtuse angles of the lateral crystals are each 

 replaced by the form t'-2£, giving a reentrant angle at x of 

 40° 57' 6". The approximated faces of this form are alone 

 strongly developed and make a trumpet-shaped cavity for the 

 reception of the wedge. The curved basal face in the free 

 crystals mentioned above were formed by the blunt point of 

 the arrow-head and by the corresponding faces of i-2J on the 

 outer obtuse angles below at a" and a'. The tendency of the 

 mineral to form these strange triplets seems to depend upon 

 the greater intensity of the crystallizing force in the direction 



