8 PROCEEDINGS OF THE NATIONAL MUSEUM vol.73 



bohedral crystals of poor luster and imperfect form as the first de- 

 posit in cavities containing later mesolite, pseudomesolite and stil- 

 bite. In other very similar specimens the calcite has steep scaleno- 

 liedral form. The dense fibrous mesolite crust closely coats the calcite 

 and, when broken away preserves perfect molds of the calcite 

 crystals. Some of these molds are 4 or 5 centimeters on an edge. 

 The calcite is colorless to pale brownish and when broken shows 

 brilliant cleavage surfaces. In a lot of specimens which contain 

 broad crusts of stilbite such rhombohedral crystals of calcite occur 

 both under and above the stilbite crust in such manner as to indicate 

 that the calcite is of two generations. 



Another small lot of calcite specimens consists of arborescent 

 masses of very small, steep rhombohedra aggregated in parallel posi- 

 tion. The calcite is white except where stained by soil or algae. 

 The only associated mineral is pseudomesolite in scattered cottony 

 tufts resting upon and evidently later than the calcite. 



HEDLANDITK 



In the specimens in which thomsonite occurs as crusts lining large 

 open spaces in a slaggy, often highly vesicular and altered basalt, 

 the smaller cavities, up to 5 millimeters across, are lined with a crust 

 about 0.5 millimeter thick of opaque-looking white crystals of another 

 mineral. Most of the smaller cavities have open centers but a few 

 of them have a solid central filling of bladed thomsonite. The 

 symmetry of the white crystals could not be made out. Some of 

 them look like chabazite twins, others look like small trapezohedra 

 of analcite, and still others suggest portions of complex phillipsite- 

 like twins. Although lustrous without, these crystals are porous 

 within and somewhat skeletal as though partly dissolved out. Fre- 

 quently the crust rests on a thin layer of waxy greenish clay. When 

 the white crystals are crushed and examined under the microscope 

 the grains lie on a perfect cleavage face which is perpendicular to 

 the acute bisectrix. The mineral is biaxial and positive, with 2V 

 small to medium. An occasional grain shows a dividing line on 

 either side of which the mineral differs in extinction showing that the 

 crystals are more or less sectored. The index of refraction, y8, is 

 1.500. 



These optical properties are in close agreement with heulandite 

 although the mineral looks unlike heulandite in the specimen. The 

 material is granular and finely aggregated making optical measure- 

 ments difficult. It seems probable that this is heulandite which is 

 secondary and pseudomorphous after some other zeolite, possibly 

 harmotome or phillipsite. Careful qualitative tests gave negative 

 results for potassium and barium. In age relations this pseudomor- 

 phous heulandite is known to be younger than the greenish clay 

 mineral and older than the thomsonite. 



