. K. Emerson — The Berry > hi f J //?,■< ami its Minerals. 



PREHNITE. — a. In fissures at Cheapside, south of the river. — 

 Prehnite occurs here most abundantly and always as the oldest 

 mineral in the veins in which it appears. That the veins where 

 it is absent have been filled at a later period and at a lower 

 tare is evident from the fact that in these the vein-walls 

 are quite as fresh as the body of the rock, while in the prehnite 

 veins the walls are deeply decomposed, often to a depth of sev- 

 eral centimeters into a rusty vesicular mass, which has been 

 filled with massive prehnite, forming a rock nearly as compact 

 as the trap itself. Similarly many detached fragments of the 

 trap have been thoroughly decomposed and in the same way 

 filled with massive prehnite. Under the microscope the mineral 

 is here seen to be made up of fibers variously matted and inter- 

 laced and intermingled with the remains of the trap, and much 

 of it exactly resembles chlorastrolite. In other specimens the 

 oldest layer of the mineral is jet-l>laek to deep oil-green, pol- 

 ished and" often slicken sided and gashed, the color being due to 

 the thorough impregnation of the prehnite with <! 

 The motion of the rock walls has also at times broken up the 

 prehnite into sheets which are slipped over each other variously 

 and re-cemented by prehnite. Wherever the mineral is hind- 

 ered in its growth it shows a strong tendency to take on these 

 fibrous forms which seems to me to depend upon a greater 

 energy of the crystallizing force in the direction of the long 

 ixis ; upon which depends also the curvature of the 

 faces so eommon in the species. Generally these fibers are 

 quite large, peculiarly riqid and in large numbers parallel to 

 each other, in on< slickensided piece the fibers of black preh- 

 nite, all straight and e slight angle to the 

 surface' of the trap, seem as if combed into this position by the 

 movement of the walls and being jet-black from enclosed dia- 

 bantite, resemble in appearance seams of fibrous hornblende or 



sparent and 

 They are apparently 

 always elongated in the direction of the long horizontal axis 

 and bounded by the planes 0, il, i-%. At times the satiny 

 luster is reflected from a large group of the needles at once, and 

 they are seen when magnified to be in juxtaposition, and form- 

 ing each group for its nes of junc- 

 tion being represented in the larger crystals by the striation 

 parallel ; > the long axis. At times the little groups ran under 

 and over each other, or joined at their ends under angles of 80° 



