. K. Emerson — The Deer field Dyke and its Minerals. 



The phenomena here 



to find their e: 



penetration of the lateral \ 



The similar twins of prehnite from Farmington, Conn., which 

 occur in the same triassic trap as the Deerfield crystals, were 

 described by M. DesCloizeaux,* and have been recently the 

 subject of discussion by the same author f and by M. Mallard:}: 

 because of their optical peculiarities. They do not seem from 

 the descriptions to present the spindle shape described above, 

 and in cross-section they differ from the Deerfield crystals in 

 one important particular. 



In the Farmington crystals the sloping faces (F in fig. 1) 

 make an angle of 100° with each other, and the termination of 

 ] is thus bounded by a threefold repetition of the 

 face i-l, while in the Deerfield forms the corresponding angle of 

 the two sloping faces measured over the single exposed face of 

 the central crystal is 80° (DbD, fig. 1), and the three faces 

 have the formulas i-i, i-l, i-i. 



ring that 



in the Farmington c 

 of the arrow-headed twin having the 

 re-entrant angle has its acute angles 

 truncated by i-l to form the sloping 

 sides, while in the Deerfield forms 

 the other end is developed, and the 

 outer obtuse angles truncated by i-i, 

 as in the annexed figure. 



It seems to me simpler to say 

 that the interpenetration of the two 

 is so complex and at the same 



re the crystal which is turned to 

 id is truncated by i-i (F in fig. 1), 

 ch at right angles to V is turned to 

 the left predominates on the same (the right) side, and deter- 

 mines the truncation by i-i (D in fig. 1), at right angles to 

 axis a"; while in the forms curvi ' 

 those with facetted faces there is 

 sition from the one to the other. 



This would also harmonize very well with the complex 

 tical results detailed in the above papers, and especially 

 the dispersion tournante signalized by M. DesCloizeaux foi 

 Farmington crystals. 



If the sloping faces in the Deerfield crystals like those ol 



* Min, p. 451, Atlas XXIX, 167 bis, 1862. 





