B. K Emerson— The Deerfield Dyke and its Minerals. 273 



of the many crystals which 

 i base, only a few survive and grow up into 

 the free space by prep to the face i '/ at, right 



angles to the long- axis, and are soon twinned so as to allow a 

 third crystal to wedge in between them and grow by the devel- 

 opment of the same face. All three grow thus predominantly 

 in the same direction and expose and add to only the single 

 crystalline face, and the crystal expands in growing like the 

 top of a growing tree. The common vertical axis of the three 

 crystals is bent thus into a circle. 



When examined under the polarizing microscope, the central 

 crystal, though showing a strong vertical and a faint horizontal 

 striation, acts as a single crystal. The two flanking crystals 

 present through a whole revolution under the Nicols a complex 

 lattice-work of brilliant colors with two predominant positions 

 of maximum extinction, or rather of extinction of the greatest 

 number of the narrow bands and wedges of color in the field, 

 at angles of 40° and 50° on either side of the suture ; that is, 

 when parallel to the two sets of axes a', &', and a" b" of the two 

 lateral crystals. The narrow wedges of color often repeated 

 jnany times, placed parallel to each other, extinguishing the 

 light together and bounded by lines making an angle of 10° 

 with each other, are especially peculiar. 



Of course the extinction of the light on the right hand of 

 f h«' suture parallel to the axis a" can alone be referred to the 

 right hand crvslal. and the . extinction at an angle of 10° with 

 !lli * and on the same side of the suture, that is parallel to the 

 axis of the left hand crystal, must be referred to this latter. So 

 that the twin is not formed simply by the approximation of the 

 two parts along the common suture plane, but by the interpen- 

 etration also of each by ihe other in narrow, and, as it were, 

 interwoven bands, as ^"represented schematically in the figure 

 —much more regularly, of course, than in nature. 



The two principal striatums in the lateral prisms make an 

 angle of 40° with the central suture— that, is, an angle of 80° 

 with each other. 



It follows, taking the prism on the right (fig. 2) for example, 



of this prism, the ot , •■■ axis h" (if the op- 



posite, ;iIl d in fact both striatums seem in the plainest manner 



Traces exist also, very faint indeed, of two other sets of stria- 

 tions at right angles respectively to those already described, 



winch are identical with the delicate horizontal lining of the 

 central crystal, and which combined with those first described 

 produce the long wedge-shaped blades with angle of 10° 



described above." This second striation is indicated in the 



