B. K. Emerson — The Deerjield Dyke and its Minerals. 357 



generally in separate, narrow fissures with coin;, 

 fresh walls, associated only with heulandite. chabazite, 

 calcite, pyrite and diabantite. but it is found sometimes 

 resting upon prehnite. It appears (a) in circular forms 

 with radiated structure 10-20 mm in diameter and up to 

 2™* thick, either spread separately or aggregated so thickly as 

 to cover the whole surface, or in small hemispheres. Farther 

 south, opposite Deerfield, fine globular forms as large as a mar- 

 ble rest upon the gashed quartz, and secondly (b) in small, stout 

 scopiform crystals /->, /'■',. o. of dull yellow color. The first form 

 occurs associated with calcite, and rarely chabazite. The second 

 with heulandite and chabazite, and I examined a flat surface of 

 the trap, four feet square, one-half of which was covered with 

 -rilbite, at first thickly covering the surface, then sep- 

 arating into distinct discs, and then quite suddenly replaced 

 oy heulandite with which was associated chabazite, and rarely 

 the small prismatic crystals of stilbite. A third form of stil- 

 hte (c) is in interlac'-d crystals. ■±""" long, perfectly pellucid, 

 with even, highly-polished faces i-i, i\ o. Under the micro- 

 scope the mineral shows a fine, rigid lining parallel to o which 

 seems to mark lines of growth, and has no effect upon the 

 »d. At right angles to this run (a) long lines of flat 

 water cavities, often negative forms in whole or part, or such 

 forms many times repeated and indicating quite rapid crystal- 

 lization, and (b) sharply marked lines of multiple twinning, 

 the whole crystal being made up of fine plates which increase 

 10 number from below upward, new plates being inl- 

 and old ones obliterated as in a compound coral. The new 

 plates sometimes appear as points and increase upward with 

 curved faces, and sometimes the old crystal develops a P-face 

 upon which the new one is based. 



Products of the decomposition of stilbite. fi 

 —Many broad surfaces of the trap are covered with 

 snow-white stellate patches of stilbite which show all stages of 

 the decomposition of the mineral into kaolin (?). In specimens 

 •ecome snow-white and opaque the kaolin can be 

 seen under the microscope in minute rounded scales and aggre- 

 gations of these into beaded lines which are crowded between 

 e latter showing no signs of 

 change but remaining limpid and polarizing apparently as viv- 

 1( % as in the freshest specimen. In other pieces where the 

 change is nearly complete the mass shows only aggregate 

 >n, and only traces of the stilbite remain. 

 Heulandite.— This mineral occurs quite abundantly at the 

 new cutting in small, stout prisms, /',. 2/, -2t up to 2 mm in 

 'ength with sharp edges and no indication of other faces. It is 

 almost always coated with a thin varnish of limonite, while the 



