B. K. Emerson — The Deerfield Dyke and its Minerals. 271 



and 100°, as if there were some trace of twinning on 7, as in 

 the spindle-shaped crystals described later. It seemed like a 

 model of the complex striation seen in the latter. 



The prehnite also occurs in many drusy cavities covered 

 with small distinct crystals, pale green, emerald-green to yel- 

 low, and in delicate emerald-green " roses " produced by the 

 ran I ti j 'le twinning of the same form 0, i-i, i-l, and in stout, square 

 prisms with base and two sides convex, the two remaining 

 sides concave. The more common botryoidal forms are scarcely 



By far the most abundant, finely-crystallized and peculiar 

 form, is a stout double cone or spindle, the two cones joining 

 base to base with an angle of 80°, or separated by a 

 lich may become as wide as the conical 

 faces by which it is bounded ; rarely this face is replaced by 

 a reentrant angle of 41°. These three faces are physically 

 unlike, the two sloping ones being sometimes smooth and 

 polished, at others mosaic-like, the equatorial plane being 

 oftentimes milled as regularly as a coin, by the oscillatory 

 repetition of I and %•%; and finally the conical faces become 

 rarely convex in the direction from" the apex to the base, pro- 

 small globular forms. These cones are laid usually 

 parallel to the surface on which they rest— the 

 axes pointing in all directions in this plane — and fused together 

 so that only a fraction of each one is distinct, though they often 

 stand out so that half or three-quarters of the circumference is 

 visible, and in this way completely cover broad surfaces with a 

 splendid crystallization. In color, they range from white and 

 nearly pellucid to pale celandine green, and rarely to pale rose 

 color in the smaller spindles with polished sides; to deep clear 

 apple-green in the largest cones ; and in other forms with very 

 i : . Several pieces a 

 foot square were obtained covered with the finest crystals. In 

 a single instance the crystals of this form have spread over cal- 

 cite, and this having been removed, they presented a group of 

 scarcely adhering individuals,— the segments of cones bounded 

 below by a single, saddle-shaped face. 



The internal structure of these crystals is quite peculiar. The 

 pearly basal cleavage passes inward in the direction of a verti- 

 cal section of the cone, and plates cut in this direction and con- 

 tinued down into the trap show the fibrous prehnite at the 

 base felted together with much decomposed trap, above grow- 

 ing purer and forming distinct crystals, with doubly striated 

 cleavage faces, which are twinned along a distinct suture and 

 bounded outwardly by irregular planes of contact. After each 

 crystal has risen above the general level to form the segment 

 of a double cone, the suture forks with an angle of about 41° 



ducing i 

 with the 



